vs_open: Ensure img_info is non-null

The tsk_vs_open function was assuming non-null input.  This caused a
segfault in the dos opening function in a case where tsk_img_open's
results weren't checked.

I missed that check in early development on an app.  I probably won't
be the last to do that.

Signed-off-by: Alex Nelson <ajnelson@cs.ucsc.edu>