yet another memcpy -> sk_careful_memcpy

will fix
https://uberchromegw.corp.google.com/i/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug-ASAN/builds/3834

No API changes.
TBR=reed@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/1514923003
1 file changed