Assert that either both or neither pointer are nullptr

If called by QMetaType::canConvert with two nullptr values,
the QMETATYPE_CONVERTER_ASSIGN macro will expand to code
dereferencing both 'to' and 'from' pointers.

Assert that others callers provide two valid pointers.

Fixes static analyzer warning 02dc34cc2ad1d4c3c6e55b44e08983f2

Pick-to: 6.1
Change-Id: I24de914faa25dc7cb1da5eae09a125506caac389
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qmetatype.cpp

@@ -857,8 +857,12 @@ static const struct : QMetaTypeModuleHelper
     {
         Q_ASSERT(fromTypeId != toTypeId);
 
+        // canConvert calls with two nullptr
         bool onlyCheck = (from == nullptr && to == nullptr);
 
+        // other callers must provide two valid pointers
+        Q_ASSERT(onlyCheck || (bool(from) && bool(to)));
+
         using Char = char;
         using SChar = signed char;
         using UChar = unsigned char;
@@ -2159,12 +2163,18 @@ static bool convertQObject(QMetaType fromType, const void *from, QMetaType toTyp
 
     Converts the object at \a from from \a fromTypeId to the preallocated space at \a to
     typed \a toTypeId. Returns \c true, if the conversion succeeded, otherwise false.
+
+    Both \a from and \a to have to be valid pointers.
+
     \since 5.2
 */
 
 /*!
     Converts the object at \a from from \a fromType to the preallocated space at \a to
     typed \a toType. Returns \c true, if the conversion succeeded, otherwise false.
+
+    Both \a from and \a to have to be valid pointers.
+
     \since 5.2
 */
 bool QMetaType::convert(QMetaType fromType, const void *from, QMetaType toType, void *to)