1.16.3

A few minor undefined behavior fixes. Most important one was in the error metric, which could have overflowed on very extreme colors, possibly causing quality to lower than it should have been. In practice, we never noticed any issues, but it's fixed.

The others were either harmless (passing NULL to memcpy() with size=0), or not really an issue but worth fixing or disabling (a few unaligned reads on x86/x64 which address sanitizer reports - not we always disabled unaligned reads in WASM builds).