Fix off-by-one bug in XactLogCommitRecord

Commit 1eb6d652 introduced zeroed alignment bytes in the GID field
of commit/abort WAL records.  Fixup commit cf5a1890 later changed
that representation into a regular cstring with a single terminating
zero byte, but it also introduced an off-by-one mistake.  Fix that.

Author: Nikhil Sontakke
Reported-by: Nikhil Sontakke
Discussion: https://postgr.es/m/CAMGcDxey6dG1DP34_tJMoWPcp5sPJUAL4K5CayUUXLQSx2GQpA@mail.gmail.com

src/backend/access/transam/xact.c

@@ -5365,7 +5365,7 @@ XactLogCommitRecord(TimestampTz commit_time,
 	{
 		XLogRegisterData((char *) (&xl_twophase), sizeof(xl_xact_twophase));
 		if (xl_xinfo.xinfo & XACT_XINFO_HAS_GID)
-			XLogRegisterData((char *) twophase_gid, strlen(twophase_gid));
+			XLogRegisterData((char *) twophase_gid, strlen(twophase_gid) + 1);
 	}
 
 	if (xl_xinfo.xinfo & XACT_XINFO_HAS_ORIGIN)