esp8266/modnetwork: Allow to press ctrl-C while scan() is running.

Ctrl-C will raise a KeyboardInterrupt and stop the scan (although it will
continue to run in the background, it won't report anything).  If
interrupted, and another scan() is started before the old one completes
in the background, then the second scan will fail with an OSError.

esp8266/modnetwork.c

@@ -172,7 +172,17 @@ STATIC mp_obj_t esp_scan(mp_obj_t self_in) {
     mp_obj_t list = mp_obj_new_list(0, NULL);
     esp_scan_list = &list;
     wifi_station_scan(NULL, (scan_done_cb_t)esp_scan_cb);
-    ETS_POLL_WHILE(esp_scan_list != NULL);
+    while (esp_scan_list != NULL) {
+        // our esp_scan_cb is called via ets_loop_iter so it's safe to set the
+        // esp_scan_list variable to NULL without disabling interrupts
+        if (MP_STATE_VM(mp_pending_exception) != NULL) {
+            esp_scan_list = NULL;
+            mp_obj_t obj = MP_STATE_VM(mp_pending_exception);
+            MP_STATE_VM(mp_pending_exception) = MP_OBJ_NULL;
+            nlr_raise(obj);
+        }
+        ets_loop_iter();
+    }
     if (list == MP_OBJ_NULL) {
         nlr_raise(mp_obj_new_exception_msg(&mp_type_OSError, "scan failed"));
     }