Don't trigger ASan use-after-poison while checking for live pointers

Don't trigger ASan use-after-poison while checking for live pointers

* src/alloc.c (live_string_holding, live_cons_holding)
(live_symbol_holding, live_float_holding) [GC_ASAN_POISON_OBJECTS]:
When compiling with address sanitization and GC poisoning support,
return NULL if the passed address is poisoned, or if the Lisp object
it resides in is poisoned. This fixes a rare use-after-poison abort
when these functions are called on a pointer that might be referring
to a now dead/sweep object.