Assert that either both or neither pointer are nullptr

Identical change to qmetatype.cpp

Fixes static analyzer warning 0267bc9b3ba521cf8bf0a7fea8981ee5

Pick-to: 6.1
Change-Id: Id6219f5025d703dd43b1742a067aa934d6aacd8c
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/gui/kernel/qguivariant.cpp

@@ -117,6 +117,8 @@ static const struct : QMetaTypeModuleHelper
         Q_ASSERT(fromTypeId != toTypeId);
 
         bool onlyCheck = (from == nullptr && to == nullptr);
+        // either two nullptrs from canConvert, or two valid pointers
+        Q_ASSERT(onlyCheck || (bool(from) && bool(to)));
 
         using Int = int;
         switch (makePair(toTypeId, fromTypeId)) {