comdlg32: Avoid possible NULL pointer dereference (Coverity)

[Kai Blin]

As the code checks for ofnW->lpstrFile not being NULL, it seems like
PathFindFileNameW() and PathFindExtensionW() don't make sense on NULL
ofnW->lpstrFile. If offset and extension are not updated, it doesn't make
sense to call UpdateResults. So just abort if ofnW->lpstrFile is NULL.

This fixes Coverity CID 108
---
 dlls/comdlg32/filedlg31.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
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