mount_image.c: Don't assume inode is resolved

mount_image.c:  Don't assume inode is resolved

The code should no longer assume that all streams have been resolved
before mounting, since now errors are issued when accessing files with
missing streams.

However this line still appeared to work correctly because FUSE seems to
always call getattr(), which results in stream resolution in
wim_pathname_to_stream(), before unlinking.