jit-builtins.c: Fix segfault on unsupported builtins

jit-builtins.c: Fix segfault on unsupported builtins

gcc/jit/ChangeLog:
	* jit-builtins.c
	(gcc::jit::recording::builtins_manager::get_builtin_function):
	Check for NULL return from make_builtin_function.
	(gcc::jit::recording::builtins_manager::make_builtin_function):
	Check for NULL return from get_type.

From-SVN: r218239