Fix incorrect snprintf() limit.

Typo in commit 7cbee7c0.  No practical effect since the buffer should
never actually be overrun, but various compilers and static analyzers will
whine about it.

Petr Jelinek

src/backend/access/transam/xlog.c

@@ -7279,7 +7279,7 @@ StartupXLOG(void)
 				char		partialpath[MAXPGPATH];
 
 				XLogFilePath(origpath, EndOfLogTLI, endLogSegNo);
-				snprintf(partialfname, MAXPGPATH, "%s.partial", origfname);
+				snprintf(partialfname, MAXFNAMELEN, "%s.partial", origfname);
 				snprintf(partialpath, MAXPGPATH, "%s.partial", origpath);
 
 				/*