Check for some potential subheap allocation failures.

src/gpu/vk/GrVkMemory.h

 /*
 * Copyright 2015 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */
 
 #ifndef GrVkMemory_DEFINED
 #define GrVkMemory_DEFINED
 
@@ skipping 82 matching lines @@
         }
     }
 
     ~GrVkHeap();
 
     VkDeviceSize allocSize() const { return fAllocSize; }
     VkDeviceSize usedSize() const { return fUsedSize; }
 
     bool alloc(VkDeviceSize size, VkDeviceSize alignment, uint32_t memoryTypeIndex, 
                GrVkAlloc* alloc) {
+        SkASSERT(size > 0);
         return (*this.*fAllocFunc)(size, alignment, memoryTypeIndex, alloc);
     }
     bool free(const GrVkAlloc& alloc);
 
 private:
     typedef bool (GrVkHeap::*AllocFunc)(VkDeviceSize size, VkDeviceSize alignment, 
                                         uint32_t memoryTypeIndex, GrVkAlloc* alloc);
 
     bool subAlloc(VkDeviceSize size, VkDeviceSize alignment, 
                   uint32_t memoryTypeIndex, GrVkAlloc* alloc);
     bool singleAlloc(VkDeviceSize size, VkDeviceSize alignment,
                      uint32_t memoryTypeIndex, GrVkAlloc* alloc);
 
     const GrVkGpu*         fGpu;
     VkDeviceSize           fSubHeapSize;
     VkDeviceSize           fAllocSize;
     VkDeviceSize           fUsedSize;
     AllocFunc              fAllocFunc;
     SkTArray<SkAutoTDelete<GrVkSubHeap>> fSubHeaps;
 };
 #endif