Only call flush in pipe mode.

It fixes a crash with a socket in top mode.

Ex:
# pmcstat -R 127.0.0.1:8080 -T -w1
then
# pmcstat -n1 -Sclock.prof -Slock.failed -O 127.0.0.1:8080

MFC after:	1 week
Sponsored by:	Stormshield

usr.sbin/pmcstat/pmcstat.c

@@ -1350,6 +1350,7 @@ main(int argc, char **argv)
 			}
 			/* print out counting PMCs */
 			if ((args.pa_flags & FLAG_DO_TOP) &&
+			    (args.pa_flags & FLAG_HAS_PIPE) &&
 			     pmc_flush_logfile() == 0)
 				do_read = 1;
 			do_print = 1;