Reserve byte for NUL terminator and report errors consistently in xmlBuf and xmlBuffer

This is a follow-up to commit 6c283d83.

* buf.c:
(xmlBufGrowInternal):
- Call xmlBufMemoryError() when the buffer size would overflow.
- Account for NUL terminator byte when using XML_MAX_TEXT_LENGTH.
- Do not include NUL terminator byte when returning length.
(xmlBufAdd):
- Call xmlBufMemoryError() when the buffer size would overflow.

* tree.c:
(xmlBufferGrow):
- Call xmlTreeErrMemory() when the buffer size would overflow.
- Do not include NUL terminator byte when returning length.
(xmlBufferResize):
- Update error message in xmlTreeErrMemory() to be consistent
  with other similar messages.
(xmlBufferAdd):
- Call xmlTreeErrMemory() when the buffer size would overflow.
(xmlBufferAddHead):
- Add overflow checks similar to those in xmlBufferAdd().

buf.c

@@ -436,9 +436,11 @@ xmlBufGrowInternal(xmlBufPtr buf, size_t len) {
 
     if (buf->alloc == XML_BUFFER_ALLOC_IMMUTABLE) return(0);
     if (len < buf->size - buf->use)
-        return(buf->size - buf->use);
+        return(buf->size - buf->use - 1);
-    if (len > SIZE_MAX - buf->use)
+    if (len >= SIZE_MAX - buf->use) {
+        xmlBufMemoryError(buf, "growing buffer past SIZE_MAX");
         return(0);
+    }
 
     if (buf->size > (size_t) len) {
         size = buf->size > SIZE_MAX / 2 ? SIZE_MAX : buf->size * 2;
@@ -451,7 +453,7 @@ xmlBufGrowInternal(xmlBufPtr buf, size_t len) {
         /*
 	 * Used to provide parsing limits
 	 */
-        if ((buf->use + len >= XML_MAX_TEXT_LENGTH) ||
+        if ((buf->use + len + 1 >= XML_MAX_TEXT_LENGTH) ||
 	    (buf->size >= XML_MAX_TEXT_LENGTH)) {
 	    xmlBufMemoryError(buf, "buffer error: text too long\n");
 	    return(0);
@@ -479,7 +481,7 @@ xmlBufGrowInternal(xmlBufPtr buf, size_t len) {
     }
     buf->size = size;
     UPDATE_COMPAT(buf)
-    return(buf->size - buf->use);
+    return(buf->size - buf->use - 1);
 }
 
 /**
@@ -839,9 +841,12 @@ xmlBufAdd(xmlBufPtr buf, const xmlChar *str, int len) {
     if (len < 0) return -1;
     if (len == 0) return 0;
 
+    /* Note that both buf->size and buf->use can be zero here. */
     if ((size_t) len >= buf->size - buf->use) {
-        if ((size_t) len >= SIZE_MAX - buf->use)
+        if ((size_t) len >= SIZE_MAX - buf->use) {
+            xmlBufMemoryError(buf, "growing buffer past SIZE_MAX");
             return(-1);
+        }
         needSize = buf->use + len + 1;
 	if (buf->alloc == XML_BUFFER_ALLOC_BOUNDED) {
 	    /*

tree.c

@@ -7371,8 +7371,10 @@ xmlBufferGrow(xmlBufferPtr buf, unsigned int len) {
     if (buf->alloc == XML_BUFFER_ALLOC_IMMUTABLE) return(0);
     if (len < buf->size - buf->use)
         return(0);
-    if (len > UINT_MAX - buf->use)
+    if (len >= UINT_MAX - buf->use) {
+        xmlTreeErrMemory("growing buffer past UINT_MAX");
         return(-1);
+    }
 
     if (buf->size > (size_t) len) {
         size = buf->size > UINT_MAX / 2 ? UINT_MAX : buf->size * 2;
@@ -7400,7 +7402,7 @@ xmlBufferGrow(xmlBufferPtr buf, unsigned int len) {
 	buf->content = newbuf;
     }
     buf->size = size;
-    return(buf->size - buf->use);
+    return(buf->size - buf->use - 1);
 }
 
 /**
@@ -7497,7 +7499,7 @@ xmlBufferResize(xmlBufferPtr buf, unsigned int size)
         return 1;
 
     if (size > UINT_MAX - 10) {
-        xmlTreeErrMemory("growing buffer");
+        xmlTreeErrMemory("growing buffer past UINT_MAX");
         return 0;
     }
 
@@ -7625,9 +7627,12 @@ xmlBufferAdd(xmlBufferPtr buf, const xmlChar *str, int len) {
     if (len < 0) return -1;
     if (len == 0) return 0;
 
+    /* Note that both buf->size and buf->use can be zero here. */
     if ((unsigned) len >= buf->size - buf->use) {
-        if ((unsigned) len >= UINT_MAX - buf->use)
+        if ((unsigned) len >= UINT_MAX - buf->use) {
+            xmlTreeErrMemory("growing buffer past UINT_MAX");
             return XML_ERR_NO_MEMORY;
+        }
         needSize = buf->use + len + 1;
         if (!xmlBufferResize(buf, needSize)){
 	    xmlTreeErrMemory("growing buffer");
@@ -7696,8 +7701,13 @@ xmlBufferAddHead(xmlBufferPtr buf, const xmlChar *str, int len) {
 	    return(0);
 	}
     }
-    needSize = buf->use + len + 2;
+    /* Note that both buf->size and buf->use can be zero here. */
-    if (needSize > buf->size){
+    if ((unsigned) len >= buf->size - buf->use) {
+        if ((unsigned) len >= UINT_MAX - buf->use) {
+            xmlTreeErrMemory("growing buffer past UINT_MAX");
+            return(-1);
+        }
+        needSize = buf->use + len + 1;
         if (!xmlBufferResize(buf, needSize)){
 	    xmlTreeErrMemory("growing buffer");
             return XML_ERR_NO_MEMORY;