QgsXmlUtils::readFlagAttribute(): fix use-after-free

Found with Valgrind

```const char* c_str = qstr.toUtf8().data()``` is invalid
since the QByteArray returned by toUtf8() is destroyed at the
end of the expression, letting c_str point to freed memory

On the contrary ```foo(qstr.toUtf8().data())``` is valid since
the temporary object is destroyed only after foo invokation.

src/core/qgsxmlutils.h

@@ -97,8 +97,7 @@ class CORE_EXPORT QgsXmlUtils
       if ( metaEnum.isValid() )
       {
         bool ok = false;
-        const char *vs = sourceCategoriesStr.toUtf8().data();
-        int newValue = metaEnum.keysToValue( vs, &ok );
+        int newValue = metaEnum.keysToValue( sourceCategoriesStr.toUtf8().data(), &ok );
         if ( ok )
           value = static_cast<T>( newValue );
       }