analyzer: add ctxt to fill_region/zero_fill_region

analyzer: add ctxt to fill_region/zero_fill_region

I noticed that region_model's fill_region/zero_fill_region member
functions weren't checking that the write to the region was valid.

Fixed thusly.

gcc/analyzer/ChangeLog:
	* kf.cc (kf_calloc::impl_call_pre): Pass ctxt to zero_fill_region.
	(kf_memset::impl_call_pre): Move responsibility for calling
	check_region_for_write to fill_region.
	* region-model.cc (region_model::on_assignment): Pass ctxt to
	zero_fill_region.
	(region_model::fill_region): Add "ctxt" param, using it to call
	check_region_for_write.
	(region_model::zero_fill_region): Likewise.
	* region-model.h (region_model::fill_region): Add "ctxt" param.
	(region_model::zero_fill_region): Likewise.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/analyzer_cpython_plugin.c: Pass ctxt to
	zero_fill_region.

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