py/vm: Don't release the GIL if the scheduler is locked.

The scheduler being locked general means we are running a scheduled
function, and switching to another thread violates that, so don't switch in
such a case (even though we technically could).

And if we are running a scheduled function then we want to finish it ASAP,
so we shouldn't switch to another thread.

Furthermore, ports with threading enabled will lock the scheduler during a
hard IRQ, and this patch to the VM will make sure that threads are not
switched during a hard IRQ (which would crash the VM).

py/vm.c

@@ -1301,8 +1301,14 @@ pending_exception_check:
                 #else
                 {
                 #endif
+                    #if MICROPY_ENABLE_SCHEDULER
+                    // can only switch threads if the scheduler is unlocked
+                    if (MP_STATE_VM(sched_state) == MP_SCHED_IDLE)
+                    #endif
+                    {
                     MP_THREAD_GIL_EXIT();
                     MP_THREAD_GIL_ENTER();
+                    }
                 }
                 #endif