pytime: add _PyTime_check_mul_overflow() macro to avoid undefined behaviour

Overflow test in test_FromSecondsObject() fails on FreeBSD 10.0 buildbot which
uses clang. clang implements more aggressive optimization which gives
different result than GCC on undefined behaviours.

Check if a multiplication will overflow, instead of checking if a
multiplicatin had overflowed, to avoid undefined behaviour.

Add also debug information if the test on overflow fails.