cppcheck: Fix "known condition warning" in networkcontrol.cpp.

This warning used to be necessary as the code formerly used a
dynamic_cast to convert types. Switching to a simple assignment since
the code was casting to a parent type removed the need for this
validity test.

mythtv/programs/mythfrontend/networkcontrol.cpp

@@ -1201,8 +1201,6 @@ QString NetworkControl::processTheme( NetworkCommand* nc)
             return QString("ERROR: no top screen found!");
 
         MythUIType *currType = topScreen;
-        if (currType == nullptr)
-            return QString("ERROR: cannot cast top screen!");
 
         while (!path.isEmpty())
         {