Annotate return_overflow in mod_write() (CID #1604620) (#5437)

Annotate return_overflow in mod_write() (CID #1604620) (#5437)

This is arguably another example of trying to return a value not
representable in the function return type. It's highly unlikely that
anyone will pass a buffer of more than SSIZE_MAX bytes, but Coverity
apparently doesn't consider that.

CIDs #1604605 and #1604616 explicitly do return error values not
representable as int, but a ridiculously large buffer allocation will
fail long before anyone calls mod_write(), so we annotate.