ITS: Bugfix in its_flash_fs_delete_idx

Bug description: when deleting a asset(asset A) whose size is 0 while a
asset(asset B) whose size is not 0 locates at the same position as
asset A, from metadata, the file data of asset B is missed when copying
file data to the scratch data block.
Fix: skip updating the data block if the size of the asset to delete is
0. Only update the metadata block.

Signed-off-by: Sherry Zhang <sherry.zhang2@arm.com>
Change-Id: I7e1581a8cdce810a5c1c2188b6e54b37f9e8d708
1 file changed