btrfs-progs: don't double-close prg_fd

If scrub start discovers that scrub is already running,
we need to set prg_fd to -1 before goto out, or we'll
try to close it again in the error path.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Zach Brown <zab@redhat.com>

cmds-scrub.c

@@ -1192,6 +1192,7 @@ static int scrub_start(int argc, char **argv, int resume)
 			/* ... yes, so scrub must be running. error out */
 			fprintf(stderr, "ERROR: scrub already running\n");
 			close(prg_fd);
+			prg_fd = -1;
 			goto out;
 		}
 		/*