another null src, memcpy -> sk_careful_memcpy

https://uberchromegw.corp.google.com/i/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug-ASAN/builds/3798/steps/dm/logs/stdio

BUG=skia:

Review URL: https://codereview.chromium.org/1510903005
1 file changed