ext4: handle NULL p_ext in ext4_ext_next_allocated_block()

In ext4_ext_next_allocated_block(), the path[depth] might
have a p_ext that is NULL -- see ext4_ext_binsearch().  In
such a case, dereferencing it will crash the machine.

This patch checks for p_ext == NULL in
ext4_ext_next_allocated_block() before dereferencinging it.

Tested using a hand-crafted an inode with eh_entries == 0 in
an extent block, verified that running FIEMAP on it crashes
without this patch, works fine with it.
Signed-off-by: NCurt Wohlgemuth <curtw@google.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/extents.c

@@ -1392,7 +1392,8 @@ ext4_ext_next_allocated_block(struct ext4_ext_path *path)
 	while (depth >= 0) {
 		if (depth == path->p_depth) {
 			/* leaf */
-			if (path[depth].p_ext !=
+			if (path[depth].p_ext &&
+				path[depth].p_ext !=
 					EXT_LAST_EXTENT(path[depth].p_hdr))
 			  return le32_to_cpu(path[depth].p_ext[1].ee_block);
 		} else {