chatView: Guard calls to gtk_style_context_get_color()

Commit fe51ac273c8 changed the fallback behavior when the looked up
state mismatches the actual state; as a result, the lookup may now
trigger the GtkStyleContext::changed signal - and as we happen to do
the lookup from a ::changed handler, we set off infinite recursion
with the expected fatal consequences.
Prevent this by guarding the lookup with a save()/restore() pair.