assert() is compiled out for release builds, but we don't want to
continue running in these impossible situations.
This also resolves the "implicit fallthrough" warnings for the asserts
in switch cases.
After merging +num64, the 64-bit sanitizer builds show that Vim doesn't
buffer the user from C's UB in signed arithmetic. Upstream doesn't
appear to be [interested] in fixing the issue, so suppress UBSAN until
someone decides to fix the problem.
N.B., the problem existed before but went unnoticed since the sanitizer
builds weren't being run in 32-bit mode.
[interested]: https://groups.google.com/d/topic/vim_dev/_tqf8eQy5eA/discussion
