Fix assert in logicalmsg_desc

The assert, introduced by 9f1cf97, is intended to check if the prefix
is terminated by a \0 byte, but it has two flaws. Firstly, prefix_size
includes the \0 byte, so prefix[prefix_size] points to the byte after
the null byte. Secondly, the check ensures the byte is not equal \0,
while it should be checking the opposite.

Backpatch-through: 14
Discussion: https://postgr.es/m/b99b6101-2f14-3796-3dfa-4a6cd7d4326d@enterprisedb.com

Author: tvondra

src/backend/access/rmgrdesc/logicalmsgdesc.c

@@ -28,7 +28,7 @@ logicalmsg_desc(StringInfo buf, XLogReaderState *record)
 		char	   *message = xlrec->message + xlrec->prefix_size;
 		char	   *sep = "";
 
-		Assert(prefix[xlrec->prefix_size] != '\0');
+		Assert(prefix[xlrec->prefix_size - 1] == '\0');
 
 		appendStringInfo(buf, "%s, prefix \"%s\"; payload (%zu bytes): ",
 						 xlrec->transactional ? "transactional" : "non-transactional",