Code from rBb0cb0a785475: Texture Paint: sync changing active texture in the Node Editor to the assumed a texture node node->id would
always be an image.
That is not true though:

• could be an object (as reported here with the Point Density node)
• could be a textblock (as in the IES Texture node)

Acting on these would crash when doing BKE_image_signal on them.

Now check specifically for nodes that actually hold an image (`Image
Texture` / Environment Texture) and do nothing otherwise.
Also check if an image is actually set in these nodes (if none is, the
Image Editor is now also untouched, previously the image in the Image
Editor was "cleared" here [set to NULL] -- which does not seems very
beneficial)