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: Qu Wenruo <wqu@suse.com>
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>