AP_Arming: account for snprintf now null-terminating strings

libraries/AP_Arming/AP_Arming.cpp

@@ -125,7 +125,7 @@ void AP_Arming::check_failed(const enum AP_Arming::ArmingChecks check, bool repo
         return;
     }
     char taggedfmt[MAVLINK_MSG_STATUSTEXT_FIELD_TEXT_LEN+1];
-    hal.util->snprintf((char*)taggedfmt, sizeof(taggedfmt)-1, "PreArm: %s", fmt);
+    hal.util->snprintf(taggedfmt, sizeof(taggedfmt), "PreArm: %s", fmt);
     MAV_SEVERITY severity = check_severity(check);
     va_list arg_list;
     va_start(arg_list, fmt);