Check reallocated buffer for NULL before use

As *buf is reallocated in case CLVMD_CMD_TEST: test for NULL is needed
before printing status.
(realloc() == NULL and status != 0)

WHATS_NEW

@@ -1,5 +1,6 @@
 Version 2.02.78 - 
 ====================================
+  Check reallocated buffer for NULL before use in clvmd do_command().
   Fix memory leak when VG allocation policy in metadata is invalid.
   Ignore unrecognised allocation policy found in metadata instead of aborting.
   Factor out tag printing into _out_tags and avoid leaking string buffer.

daemons/clvmd/clvmd-command.c

@@ -169,7 +169,8 @@ int do_command(struct local_client *client, struct clvm_header *msg, int msglen,
 
 	/* Check the status of the command and return the error text */
 	if (status) {
-		*retlen = 1 + snprintf(*buf, buflen, "%s", strerror(status));
+		*retlen = 1 + (*buf) ? snprintf(*buf, buflen, "%s",
+						strerror(status)) : -1;
 	}
 
 	return status;