st-theme-node-drawing.c: test for COGL_INVALID_HANDLE 

Having a look at the possible causes of issue #2125, I noticed this in the log:
````
_st_create_shadow_material: assertion `src_texture != COGL_INVALID_HANDLE' failed
````
Following that trail, I noticed one instance where the code conspicuously does not check for COGL_INVALID_HANDLE before using a value returned from _st_create_shadow_material.