GCS_Mavlink.pde: Fix for a possible segfault after passing ARSPD_FBW_MIN

Dear APM maintainer,

When experimenting a little with the PX4 serial ports and intending to use the GCS[2] port for sending out some internal APM data, I have noticed that the APM build on PX4 crashes when the aircraft flies faster than ARSPD_FBW_MIN. This was caused by the airspeed calibration data being redistributed via all gcs channels, even though one of the gcs channels was not used (nor initialized). Hence I added one additional if cause... I hope this fix helps someone with similar troubles.

Regards,