mem_tracker: Don't try to copy data if none available

Check that call received data before trying to copy.
diff --git a/layers/mem_tracker.cpp b/layers/mem_tracker.cpp
index f2b3c91..1ae5bde 100644
--- a/layers/mem_tracker.cpp
+++ b/layers/mem_tracker.cpp
@@ -2923,7 +2923,7 @@
 {
     VkResult result = get_dispatch_table(mem_tracker_device_table_map, (void*) swapChain.handle)->GetSwapChainInfoWSI(device, swapChain, infoType, pDataSize, pData);
 
-    if (infoType == VK_SWAP_CHAIN_INFO_TYPE_IMAGES_WSI && result == VK_SUCCESS) {
+    if (infoType == VK_SWAP_CHAIN_INFO_TYPE_IMAGES_WSI && result == VK_SUCCESS && pData != NULL) {
         const size_t count = *pDataSize / sizeof(VkSwapChainImagePropertiesWSI);
         MT_SWAP_CHAIN_INFO *pInfo = swapChainMap[swapChain.handle];