analyzer: fix ICE on realloc of zeroed memory [PR104062]

gcc/analyzer/ChangeLog:
	PR analyzer/104062
	* region-model-manager.cc
	(region_model_manager::maybe_fold_sub_svalue): Avoid casting to
	NULL type when folding access to repeated svalue.

gcc/testsuite/ChangeLog:
	PR analyzer/104062
	* gcc.dg/analyzer/pr104062.c: New test.

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