Improve error checking in flex6xxx.c

rigs/kenwood/flex6xxx.c

@@ -877,6 +877,13 @@ int powersdr_get_level(RIG *rig, vfo_t vfo, setting_t level, value_t *val)
 
     case RIG_LEVEL_AF:
         n = sscanf(lvlbuf, "ZZAG%f", &val->f);
+        if (n != 1)
+        {
+            rig_debug(RIG_DEBUG_ERR, "%s: Error parsing value from lvlbuf='%s'\n",
+                      __func__, lvlbuf);
+            val->f = 0;
+            return -RIG_EPROTO;
+        }
         val->f /= 100.0;
         break;