fix a corner case in getBufferSubData range checking

third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "modules/webgl/WebGL2RenderingContextBase.h"
 
 #include "bindings/modules/v8/WebGLAny.h"
 #include "core/frame/ImageBitmap.h"
 #include "core/html/HTMLCanvasElement.h"
 #include "core/html/HTMLImageElement.h"
@@ skipping 355 matching lines @@
   WebGLBuffer* buffer = validateBufferDataTarget(funcName, target);
   if (!buffer)
     return;
   void* subBaseAddress = nullptr;
   long long subByteLength = 0;
   if (!validateSubSourceAndGetData(dstData, dstOffset, length, &subBaseAddress,
                                    &subByteLength)) {
     synthesizeGLError(GL_INVALID_VALUE, funcName, "buffer overflow");
     return;
   }
+  if (subByteLength == 0) {
+    return;
+  }
 
   void* mappedData =
       contextGL()->MapBufferRange(target, static_cast<GLintptr>(srcByteOffset),
                                   subByteLength, GL_MAP_READ_BIT);
 
   if (!mappedData)
     return;
 
   memcpy(subBaseAddress, mappedData, subByteLength);
 
@@ skipping 4103 matching lines @@
   params.skipPixels = m_unpackSkipPixels;
   params.skipRows = m_unpackSkipRows;
   if (dimension == Tex3D) {
     params.imageHeight = m_unpackImageHeight;
     params.skipImages = m_unpackSkipImages;
   }
   return params;
 }
 
 }  // namespace blink