don't crash in message expiration debug print

we would try to print the uids from the non-existing srec of unpaired
messages while preparing expiration.
this would happen only if a) MaxMessages was configured and b) new
messages appeared on the slave but we were not pushing, so it's a bit of
a corner case.

found by coverity.

src/sync.c

@@ -1360,7 +1360,7 @@ box_loaded( int sts, void *aux )
 				continue;
 			if (!(srec = tmsg->srec) || srec->uid[M] <= 0) {
 				/* We did not push the message, so it must be kept. */
-				debug( "  old pair(%d,%d) unpropagated\n", srec->uid[M], srec->uid[S] );
+				debug( "  message %d unpropagated\n", tmsg->uid );
 				todel--;
 			} else {
 				nflags = (tmsg->flags | srec->aflags[S]) & ~srec->dflags[S];