btrfs: Add an assertion to warn incorrect case in insert_inline_extent()

In insert_inline_extent(), the case that checks compressed_size > 0
and compressed_pages = NULL cannot occur, otherwise a null-pointer
dereference may occur on line 215:

     cpage = compressed_pages[i];

To catch this incorrect case, an assertion is added.
Reviewed-by: NQu Wenruo <wqu@suse.com>
Signed-off-by: NJia-Ju Bai <baijiaju1990@gmail.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -176,6 +176,9 @@ static int insert_inline_extent(struct btrfs_trans_handle *trans,
 	size_t cur_size = size;
 	unsigned long offset;
 
+	ASSERT((compressed_size > 0 && compressed_pages) ||
+	       (compressed_size == 0 && !compressed_pages));
+
 	if (compressed_size && compressed_pages)
 		cur_size = compressed_size;