Don't leak f_out file desciptor if specified more than once

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/init.c b/init.c
index a0ec914..35fbdf1 100644
--- a/init.c
+++ b/init.c
@@ -1719,6 +1719,9 @@
 			write_bw_log = 1;
 			break;
 		case 'o':
+			if (f_out)
+				fclose(f_out);
+
 			f_out = fopen(optarg, "w+");
 			if (!f_out) {
 				perror("fopen output");