AP_NavEKF2: correct return of empty prearm string

libraries/AP_NavEKF2/AP_NavEKF2_Measurements.cpp

@@ -603,6 +603,7 @@ void NavEKF2_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");
         }
     }
 }