status is unreliable, check for nan explicitly.

Losing the GPS fix often results in gpsd reporting status as ok, but
reporting nan as position. These invalid coordinates result in broken
position data (nan as position, random painting on the map) and likely
also crashes.

BUG: 312212
FIXED-IN: KDE/4.10.0;Marble/1.5.0
CCBUG: 313224
(cherry picked from commit 17b732a7)