DMU: Fixed refactoring oversight in P_ToPtr()

The semantics of looking up a Material by manifest id differ from
those of the DMU API itself. This function must either check that
the given index is non-zero before calling Materials::toManifest()
or handle the exception.

We'll test for a non-zero index specially so that the exception is
otherwise still raised as expected.

doomsday/client/src/map/p_dmu.cpp

@@ -357,7 +357,9 @@ void *P_ToPtr(int type, int index)
 
     case DMU_MATERIAL:
         /// @note @a index is 1-based.
-        return &App_Materials().toManifest(index).material();
+        if(index > 0)
+            return &App_Materials().toManifest(index).material();
+        return 0;
 
     default: {
         /// @todo Throw exception.