solid: check if the object is null before qobject_cast<T>()-ing it in return_SOLID_CALL() macro

fixes possible crashes, notably on FreeBSD the backend of which does not
implement all interfaces

Signed-off-by: Ivailo Monev <xakepa10@gmail.com>

solid/solid/soliddefs_p.h

@@ -24,8 +24,11 @@
 #include <QtCore/QObject>
 
 #define return_SOLID_CALL(Type, Object, Default, Method) \
+    if (Object == nullptr) { \
+        return Default; \
+    } \
     Type t = qobject_cast<Type>(Object); \
-    if (t!=0) \
+    if (t != nullptr) \
     { \
          return t->Method; \
     } \