Use valid `ec` for postponed job.

Postponed job can be registered from non-Ruby thread, which means
`ec` in TLS can be NULL. In this case, use main thread's `ec` instead.

See https://github.com/ruby/ruby/pull/4108
and https://github.com/ruby/ruby/pull/4336

vm_trace.c

@@ -1582,6 +1582,14 @@ postponed_job_register(rb_execution_context_t *ec, rb_vm_t *vm,
     return PJRR_SUCCESS;
 }
 
+static rb_execution_context_t *
+get_valid_ec(rb_vm_t *vm)
+{
+    rb_execution_context_t *ec = GET_EC();
+    if (ec == NULL) ec = rb_vm_main_ractor_ec(vm);
+    return ec;
+}
+
 /*
  * return 0 if job buffer is full
  * Async-signal-safe
@@ -1589,8 +1597,8 @@ postponed_job_register(rb_execution_context_t *ec, rb_vm_t *vm,
 int
 rb_postponed_job_register(unsigned int flags, rb_postponed_job_func_t func, void *data)
 {
-    rb_execution_context_t *ec = GET_EC();
-    rb_vm_t *vm = rb_ec_vm_ptr(ec);
+    rb_vm_t *vm = GET_VM();
+    rb_execution_context_t *ec = get_valid_ec(vm);
 
   begin:
     switch (postponed_job_register(ec, vm, flags, func, data, MAX_POSTPONED_JOB, vm->postponed_job_index)) {
@@ -1608,8 +1616,8 @@ rb_postponed_job_register(unsigned int flags, rb_postponed_job_func_t func, void
 int
 rb_postponed_job_register_one(unsigned int flags, rb_postponed_job_func_t func, void *data)
 {
-    rb_execution_context_t *ec = GET_EC();
-    rb_vm_t *vm = rb_ec_vm_ptr(ec);
+    rb_vm_t *vm = GET_VM();
+    rb_execution_context_t *ec = get_valid_ec(vm);
     rb_postponed_job_t *pjob;
     rb_atomic_t i, index;