hash: Fix possible startup crash with old libxslt versions

Call xmlInitParser in xmlHashCreate to make it work if the library
wasn't initialized yet.

Otherwise, exsltRegisterAll from libxslt 1.1.24 or older might cause
a crash.

See #534.

hash.c

@@ -180,6 +180,8 @@ xmlHashTablePtr
 xmlHashCreate(int size) {
     xmlHashTablePtr table;
 
+    xmlInitParser();
+
     if (size <= 0)
         size = 256;