clean up warnings in Win32 build of mmapmodule.c about cpython HOT 4 CLOSED

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Cool. Accepted & assigned back to Trent.

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Fix a few warnings in the Win32 build of mmapmodule.c. The warnings were
about comparison of singed (int) and unsigned (size_t) values. This was
patched by casting every int to size_t for comparison. This is safe because
every cast is preceded with a check guaranteeing that the int is positive.

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