Check for some potential subheap allocation failures.

src/gpu/vk/GrVkMemory.h

Index: src/gpu/vk/GrVkMemory.h
diff --git a/src/gpu/vk/GrVkMemory.h b/src/gpu/vk/GrVkMemory.h
index 417fb5a1e050c41e552f164cd58a647f4c3a18c5..44c82867218dc189be44458e47cb7ea0b5dc80f8 100644
--- a/src/gpu/vk/GrVkMemory.h
+++ b/src/gpu/vk/GrVkMemory.h
@@ -100,6 +100,7 @@ public:
 
     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);