Fix crash accessing builtins in sanitizer.def and after (PR jit/82174)

Fix crash accessing builtins in sanitizer.def and after (PR jit/82174)

Calls to gcc_jit_context_get_builtin_function that accessed builtins
in sanitizer.def and after (or failed to match any builtin) led to
a crash accessing a NULL builtin name.

The entries with the NULL name came from these lines in sanitizer.def:

  /* This has to come before all the sanitizer builtins.  */
  DEF_BUILTIN_STUB(BEGIN_SANITIZER_BUILTINS, (const char *)0)

  [...snip...]

  /* This has to come after all the sanitizer builtins.  */
  DEF_BUILTIN_STUB(END_SANITIZER_BUILTINS, (const char *)0)

This patch updates jit-builtins.c to cope with such entries, fixing the
crash.

gcc/jit/ChangeLog:
	PR jit/82174
	* jit-builtins.c (matches_builtin): Ignore entries with a NULL
	name.

gcc/testsuite/ChangeLog:
	PR jit/82174
	* jit.dg/test-error-gcc_jit_context_get_builtin_function-unknown-builtin.c:
	New test case.

From-SVN: r252769