Definitions: Fixed OBOE interpreting supplanted material anims

A typo in the definition parser's handling of the case where a
texture animation defined by the default game ANIMDEFS, replaced
by an add-on, resulted in a crash.

doomsday/client/src/def_read.cpp

@@ -1273,9 +1273,9 @@ static int DED_ReadData(ded_t* ded, const char* buffer, const char* _sourceFile)
                             // Need to allocate a new stage?
                             if(layerStage >= mat->layers[layer].stageCount.num)
                             {
-                                layerStage = DED_AddMaterialLayerStage(&mat->layers[layer]);
+                                DED_AddMaterialLayerStage(&mat->layers[layer]);
 
-                                if(mat->autoGenerated && layerStage > 1)
+                                if(mat->autoGenerated && layerStage > 0)
                                 {
                                     // When adding a new stage to an autogenerated material
                                     // initialize by copying values from the previous stage.