selftests/sched_ext: Check skeleton open failure in exit test

exit.c does not check the return value of exit__open(). If it returns
NULL, which happens when the skeleton wrapper fails to allocate the
object or open the BPF ELF, the very next line dereferences it via
SCX_ENUM_INIT(). That macro expands to SCX_ENUM_SET(), which accesses
skel->rodata, causing a NULL pointer dereference.

Every other selftest guards this with SCX_FAIL_IF(!skel, ...); exit.c
is the only one missing it.

Fixes: a5db7817af ("sched_ext: Add selftests")
Signed-off-by: Liang Luo <luoliang@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>

tools/testing/selftests/sched_ext/exit.c

@@ -31,6 +31,7 @@ static enum scx_test_status run(void *ctx)
 			continue;
 
 		skel = exit__open();
+		SCX_FAIL_IF(!skel, "Failed to open");
 		SCX_ENUM_INIT(skel);
 		skel->rodata->exit_point = tc;
 		SCX_FAIL_IF(exit__load(skel), "Failed to load skel");