Don't allow nullptr in texels array params (unless using a transfer buffer).

src/gpu/vk/GrVkGpu.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrVkGpu.h"
 
 #include "GrContextOptions.h"
@@ skipping 524 matching lines @@
 #endif
     } else {
         tex = GrVkTexture::CreateNewTexture(this, desc, lifeCycle, imageDesc);
     }
 
     if (!tex) {
         return nullptr;
     }
 
     // TODO: We're ignoring MIP levels here.
-    if (!texels.empty() && texels.begin()->fPixels) {
+    if (!texels.empty()) {
+        SkASSERT(texels.begin()->fPixels);
         if (!this->uploadTexData(tex, 0, 0, desc.fWidth, desc.fHeight, desc.fConfig,
                                  texels.begin()->fPixels, texels.begin()->fRowBytes)) {
             tex->unref();
             return nullptr;
         }
     }
 
     return tex;
 }
 
@@ skipping 863 matching lines @@
     int set_a_break_pt_here = 9;
     aglSwapBuffers(aglGetCurrentContext());
 #elif defined(SK_BUILD_FOR_WIN32)
     SwapBuf();
     int set_a_break_pt_here = 9;
     SwapBuf();
 #endif
 #endif
 }