Fix memory leak in new GUC check_hook

Commit 38e0190ced71 forgot to pfree() an allocation (freed in other
places of the same function) in only one of several spots in
check_log_min_messages().  Per Coverity.  Add that.

While at it, avoid open-coding guc_strdup().  The new coding does a
strlen() that wasn't there before, but I doubt it's measurable.

src/backend/utils/error/elog.c

@@ -2363,11 +2363,12 @@ lmm_fail:
 			appendStringInfo(&buf, ", %s", elem);
 	}
 
-	result = (char *) guc_malloc(LOG, buf.len + 1);
+	result = guc_strdup(LOG, buf.data);
 	if (!result)
+	{
+		pfree(buf.data);
 		return false;
-	memcpy(result, buf.data, buf.len);
-	result[buf.len] = '\0';
+	}
 
 	guc_free(*newval);
 	*newval = result;