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

src/xml/SkDOM.cpp

@@ -240,7 +240,7 @@ protected:
         }
         *fParentStack.push() = node;
 
-        memcpy(node->attrs(), fAttrs.begin(), attrCount * sizeof(SkDOM::Attr));
+        sk_careful_memcpy(node->attrs(), fAttrs.begin(), attrCount * sizeof(SkDOM::Attr));
         fAttrs.reset();
 
     }