AP_NavEKF3: correct return of empty prearm string

libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp

@@ -628,6 +628,7 @@ void NavEKF3_core::readGpsData()
         } else {
             // report GPS fix status
             gpsCheckStatus.bad_fix = true;
+            hal.util->snprintf(prearm_fail_string, sizeof(prearm_fail_string), "Waiting for 3D fix");
         }
     }
 }