analyzer: handle NULL inner context in region_model_context_decorator

gcc/analyzer/ChangeLog:
	* region-model.cc (region_model_context_decorator::add_event):
	Handle m_inner being NULL.
	* region-model.h (class region_model_context_decorator): Likewise.
	(annotating_context::warn): Likewise.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>