Initialize the center of the Map item

Center is initialized with center (latitude and longitude) being NAN.
Without initializing the center the Map item is not useable (navigation is not working)
and it sometimes crashes.

Change-Id: I708e9f0efd05ee0b3ed0ebc0f0cac818b6185c12
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>