fs: if search fails, return empty result set

Make sure the result set is empty before running the ioctl in case
something tries to consume the result without checking the error status.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/fs.cc

@@ -823,6 +823,8 @@ namespace crucible {
 
 		ioctl_ptr->buf_size = m_buf_size;
 
+		m_result.clear();
+
 		// Don't bother supporting V1.  Kernels that old have other problems.
 		int rv = ioctl(fd, BTRFS_IOC_TREE_SEARCH_V2, ioctl_ptr);
 		if (rv != 0) {
@@ -831,8 +833,6 @@ namespace crucible {
 
 		static_cast<btrfs_ioctl_search_key&>(*this) = ioctl_ptr->key;
 
-		m_result.clear();
-
 		size_t offset = pointer_distance(ioctl_ptr->buf, ioctl_ptr);
 		for (decltype(nr_items) i = 0; i < nr_items; ++i) {
 			BtrfsIoctlSearchHeader item;