Fix issue recycling GPU memory blocks;

When a memory block is used for host-visible memory, its mapped pointer
is tracked with the block.  If that memory is freed and later re-used
for some non-mappable memory, the pointer never gets cleared, and so
code thinks the memory is mappable and tries to use the pointer.

src/core/gpu_vk.c

@@ -2597,6 +2597,7 @@ static gpu_memory* gpu_allocate(gpu_memory_type type, VkMemoryRequirements info,
 static void gpu_release(gpu_memory* memory) {
   if (memory && --memory->refs == 0) {
     condemn(memory->handle, VK_OBJECT_TYPE_DEVICE_MEMORY);
+    memory->pointer = NULL;
     memory->handle = NULL;
 
     for (uint32_t i = 0; i < COUNTOF(state.allocators); i++) {