analyzer: fix zero-fill of calloc

analyzer: fix zero-fill of calloc

It turned out that the analyzer wasn't treating calloc regions
as zero-filled, due to binding_cluster::fill_region getting an
unknown value for the byte_size_size_sval, and thus
get_or_create_repeated_svalue returning an unknown_svalue, which
was then used to fill the region.

Fixed thusly.

gcc/analyzer/ChangeLog:
	* region-model-impl-calls.cc (region_model::impl_call_calloc): Use
	a sized_region when calling zero_fill_region.

gcc/testsuite/ChangeLog:
	* gcc.dg/analyzer/calloc-1.c: New test.

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