AP_GPS: fixed a bug in handling corrupt u-blox packets

when we have corrupt input due to loss of bytes on a UART we can end
up with a u-blox packet with zero payload bytes. When that happens we
need to bypass payload reading as otherwise we will end up keeping
reading bytes until the driver resets at 4 seconds, causing a GPS
outage.

This was causing GPS outages every few hours in copters running 3.6.7,
and was also reproduced in SITL using SIM_GPS_BYTELOSS.

libraries/AP_GPS/AP_GPS_UBLOX.cpp

@@ -467,6 +467,10 @@ AP_GPS_UBLOX::read(void)
 				goto reset;
             }
             _payload_counter = 0;                       // prepare to receive payload
+            if (_payload_length == 0) {
+                // bypass payload and go straight to checksum
+                _step++;
+            }
             break;
 
         // Receive message data