analyzer: fix state explosion on va_arg [PR106413]

Fix state explosion on va_arg when the call to va_start is in the
top-level function of the analysis.

gcc/analyzer/ChangeLog:
	PR analyzer/106413
	* varargs.cc (region_model::impl_call_va_start): Avoid iterating
	through non-existant variadic arguments by initializing the
	impl_region to "UNKNOWN" if the va_start occurs in the top-level
	function to the analysis.

gcc/testsuite/ChangeLog:
	PR analyzer/106413
	* gcc.dg/analyzer/torture/stdarg-4.c: New test.

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