btrfs-progs: corrupt-block: fix memory leak in debug_corrupt_sector()

ASAN build (make D=asan) detects a  memory leak in
btrfs-corrupt-block inside debug_corrupt_sector().

This can be reproduced by fsck/013 test case.

The cause is pretty simple, we just malloc a sector and forgot to free
it.

Issue: #806
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710

btrfs-corrupt-block.c

@@ -70,15 +70,15 @@ static int debug_corrupt_sector(struct btrfs_root *root, u64 logical, int mirror
 			if (ret < 0) {
 				errno = -ret;
 				error("cannot read bytenr %llu: %m", logical);
-				return ret;
+				goto out;
 			}
 			printf("corrupting %llu copy %d\n", logical, mirror_num);
 			memset(buf, 0, sectorsize);
 			ret = write_data_to_disk(fs_info, buf, logical, sectorsize);
 			if (ret < 0) {
 				errno = -ret;
 				error("cannot write bytenr %llu: %m", logical);
-				return ret;
+				goto out;
 			}
 		}
 
@@ -90,7 +90,8 @@ static int debug_corrupt_sector(struct btrfs_root *root, u64 logical, int mirror
 		if (mirror_num > num_copies)
 			break;
 	}
-
+out:
+	free(buf);
 	return 0;
 }