Don't try to renaming a bad (unpackable) REP packet if it doesn't exist

There still appears to be some race condition with network (Samba) FS
directory caching and opening/locking of files, so it does seem illogical that
the packet file wouldn't exist at this point (since we have the lock file
opened) but this does still happen on occasion on Vertrauen.

src/sbbs3/main.cpp

@@ -2993,7 +2993,7 @@ void event_thread(void* arg)
 					/* putuserdat? */
 					if(success) {
 						sbbs->fremove(WHERE, fname, /* log-all-errors: */true);
-					} else {
+					} else if(fexist(fname)) {
 						char badpkt[MAX_PATH+1];
 						SAFEPRINTF2(badpkt, "%s.%" PRIx64 ".bad", fname, (uint64_t)time(NULL));
 						sbbs->fremove(WHERE, badpkt);