analyzer: fix ICE on escaped unknown pointers [PR96611]

analyzer: fix ICE on escaped unknown pointers [PR96611]

PR analyzer/96611 reports an ICE within the handling for unknown
functions, when passing a pointer to something accessed via a
global pointer, after an unknown function has already been called.

The first unknown function leads to the store being flagged, so
the access to the global pointer leads to (*unknown_svalue) for
the base region of the argument to the 2nd function, and thus
*unknown_svalue being reachable by the 2nd unknown function,
triggering an assertion failure.

Handle this case by rejecting attempts to get a cluster for
the unknown pointer, fixing the ICE.

gcc/analyzer/ChangeLog:
	PR analyzer/96611
	* store.cc (store::mark_as_escaped): Reject attempts to
	get a cluster for an unknown pointer.

gcc/testsuite/ChangeLog:
	PR analyzer/96611
	* gcc.dg/analyzer/pr96611.c: New test.