Fix race condition with calling lastNMEA twice in shield_sd example.
Fix race condition with calling lastNMEA twice in shield_sd example.

ulrichard/Adafruit-GPS-Library,adafruit/Adafruit-GPS-Library,lolisa/2014-15,lolisa/2014-15,tonesandtones/Adafruit-GPS-Library,huskyroboticsteam/2014-15,huskyroboticsteam/2014-15,lolisa/2014-15,kaaLabs15/Adafruit-GPS-Library,huskyroboticsteam/2014-15