Ignore null-dereference warning

It is about
```
return result_type{ storageEntry.id, storageEntry.ref.relatedStorage };
```
but it isn't clear why the compiler thinks there's a problem.

libpkg/data/storage.cpp

@@ -1,3 +1,6 @@
+// ignore warning about "return result_type{ storageEntry.id, storageEntry.ref.relatedStorage };"
+#pragma GCC diagnostic ignored "-Wnull-dereference"
+
 #include "./storageprivate.h"
 
 #include <c++utilities/conversion/stringbuilder.h>