fix a couple flaky nonnull attribute ubsan warnings

include/core/SkTypes.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkTypes_DEFINED
 #define SkTypes_DEFINED
 
@@ skipping 82 matching lines @@
 /** Much like calloc: returns a pointer to at least size zero bytes, or NULL on failure.
  */
 SK_API extern void* sk_calloc(size_t size);
 
 /** Same as sk_calloc, but throws an exception instead of returning NULL on failure.
  */
 SK_API extern void* sk_calloc_throw(size_t size);
 
 // bzero is safer than memset, but we can't rely on it, so... sk_bzero()
 static inline void sk_bzero(void* buffer, size_t size) {
-    memset(buffer, 0, size);
+    // Please c.f. sk_careful_memcpy.  It's undefined behavior to call memset(null, 0, 0).
+    if (size) {
+        memset(buffer, 0, size);
+    }
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
 #ifdef override_GLOBAL_NEW
 #include <new>
 
 inline void* operator new(size_t size) {
     return sk_malloc_throw(size);
 }
@@ skipping 576 matching lines @@
 private:
     void*       fPtr;
     size_t      fSize;  // can be larger than the requested size (see kReuse)
     uint32_t    fStorage[(kSize + 3) >> 2];
 };
 // Can't guard the constructor because it's a template class.
 
 #endif /* C++ */
 
 #endif