fix warning: dereference of a null pointer

This fixes warning: Access to field 'next' results in a
dereference of a null pointer (loaded from variable 'prev')
[clang-analyzer-core.NullDereference]

Bug: None
Test: The warning is gone.
Change-Id: I7a839f94b9c6d56a91bfda1097ba23ff247c9db1
diff --git a/vulkan/libvulkan/debug_report.cpp b/vulkan/libvulkan/debug_report.cpp
index 0c2f138..40ba1e5 100644
--- a/vulkan/libvulkan/debug_report.cpp
+++ b/vulkan/libvulkan/debug_report.cpp
@@ -46,7 +46,8 @@
         Node* prev = &head_;
         while (prev && prev->next != node)
             prev = prev->next;
-        prev->next = node->next;
+        if (prev)
+            prev->next = node->next;
     }
 
     allocator.pfnFree(allocator.pUserData, node);