[2/2] winemenubuilder: fix stack overflow caused by invalid icon associations

[Indrek Altpere]

Fixes https://bugs.winehq.org/show_bug.cgi?id=35230
Added file existence check with stat() call before calling open_icon,
allowing winemenubuilder -a -r run to completion with Freehand 9 demo
installed.

Regards,
Indrek
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