Bug 1611837 - Allow null `data` if `dataLen` is zero. r=lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D61953

--HG--
extra : moz-landing-system : lando

dom/canvas/WebGLBuffer.cpp

@@ -183,6 +183,8 @@ void WebGLBuffer::BufferSubData(GLenum target, uint64_t dstByteOffset,
 
   ////
 
+  if (!dataLen) return;  // With validation successful, nothing else to do.
+
   const void* uploadData = data;
   if (mIndexCache) {
     const auto cachedDataBegin = (uint8_t*)mIndexCache.get() + dstByteOffset;

dom/canvas/WebGLContextBuffers.cpp

@@ -254,7 +254,7 @@ void WebGLContext::BufferData(GLenum target, uint64_t dataLen,
 
 void WebGLContext::BufferSubData(GLenum target, uint64_t dstByteOffset,
                                  uint64_t dataLen, const uint8_t* data) const {
-  MOZ_ASSERT(data);
+  MOZ_ASSERT(data || !dataLen);
   const FuncScope funcScope(*this, "bufferSubData");
   if (IsContextLost()) return;