comdlg32: Fix NULL lpstrFile uses in FILEDLG95_OnOpen

[Bruno Jesus]

Currently wine will crash if during a common dialog operation
lpstrFile == NULL && nMaxFile == 0.
Also it's possible to crash wine using a bogus input by setting
nMaxFile > 0 together with a lpstrFile == NULL.
The second crash is not present in the unicode version of the function
since it does not rely on the struct field lpstrFile as the ascii
version does.

Fixes bug #17229.
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