It's undefined behavior to pass null as src or dst to memcpy, even if len is 0.
This currently triggers -fsanitize=attribute-nonnull warnings, but also can
lead to very unexpected code generation with GCC.
sk_careful_memcpy() checks len first before calling memcpy(),
which prevents that weird undefined situation.
This allows me to mark all sanitizers as no-recover, i.e. make-the-bots-red fatal.
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug-ASAN-Trybot
BUG=skia:4641
NOTREECHECKS=true
Review URL: https://codereview.chromium.org/1510683002
cc881dafcb