clean up warnings in Win32 build of mmapmodule.c

[bdaf8532-ded6-4d4c-9bb3-51fd76040dc1]

BPO	400613
Nosy	@tim-one
Files	None: None

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields: ```python assignee = None closed_at = created_at = labels = [] title = 'clean up warnings in Win32 build of mmapmodule.c' updated_at = user = 'https://bugs.python.org/tmick' ``` bugs.python.org fields: ```python activity = actor = 'tim.peters' assignee = 'tmick' closed = True closed_date = None closer = None components = ['None'] creation = creator = 'tmick' dependencies = [] files = ['2461'] hgrepos = [] issue_num = 400613 keywords = ['patch'] message_count = 4.0 messages = ['32815', '32816', '32817', '32818'] nosy_count = 2.0 nosy_names = ['tim.peters', 'tmick'] pr_nums = [] priority = 'normal' resolution = None stage = None status = 'closed' superseder = None type = None url = 'https://bugs.python.org/issue400613' versions = [] ```

[bdaf8532-ded6-4d4c-9bb3-51fd76040dc1]

[tim-one]

Cool. Accepted & assigned back to Trent.

[bdaf8532-ded6-4d4c-9bb3-51fd76040dc1]

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[bdaf8532-ded6-4d4c-9bb3-51fd76040dc1]

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.