ext4: fix smatch warning in move_extent.c's mext_replace_branches()

Commit 2147b1a6 resulted in a new smatch warning:

> fs/ext4/move_extent.c:693 mext_replace_branches()
> 	 warn: variable dereferenced before check 'dext' (see line 683)

Fix this by adding a check to make sure dext is non-NULL before we
derefrence it.
Signed-off-by: Akria Fujita <a-fujita@rs.jp.nec.com>
[ modified by tytso to make sure an ext4_error is called ]
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/move_extent.c

@@ -681,6 +681,8 @@ mext_replace_branches(handle_t *handle, struct inode *orig_inode,
 
 	depth = ext_depth(donor_inode);
 	dext = donor_path[depth].p_ext;
+	if (unlikely(!dext))
+		goto missing_donor_extent;
 	tmp_dext = *dext;
 
 	*err = mext_calc_swap_extents(&tmp_dext, &tmp_oext, orig_off,
@@ -691,7 +693,8 @@ mext_replace_branches(handle_t *handle, struct inode *orig_inode,
 	/* Loop for the donor extents */
 	while (1) {
 		/* The extent for donor must be found. */
-		if (!dext) {
+		if (unlikely(!dext)) {
+		missing_donor_extent:
 			EXT4_ERROR_INODE(donor_inode,
 				   "The extent for donor must be found");
 			*err = -EIO;