espressif/common-hal/_bleio: int Characteristic observer properly; restore descriptor pre-check code

Author: dhalbert

ports/espressif/common-hal/_bleio/Characteristic.c

@@ -61,7 +61,7 @@ void common_hal_bleio_characteristic_construct(bleio_characteristic_obj_t *self,
     self->props = props;
     self->read_perm = read_perm;
     self->write_perm = write_perm;
-    self->observer = NULL;
+    self->observer = mp_const_none;
 
     // Map CP's property values to Nimble's flag values.
     self->flags = 0;

ports/espressif/common-hal/_bleio/Connection.c

@@ -425,6 +425,11 @@ static void discover_remote_services(bleio_connection_internal_t *self, mp_obj_t
                 ? service->end_handle
                 : next_characteristic->handle - 1;
 
+            // Pre-check if there are no descriptors to discover so descriptor discovery doesn't fail
+            if (end_handle <= characteristic->handle) {
+                continue;
+            }
+
             // Reset discovery status before starting callbacks
             _set_discovery_step_status(0);