Prevent updating item screen points when map width/height is 0
Prevent updating item screen points when map width/height is 0

This avoids creating a viewport with nan values, and removes
QPainterPath warnings

Change-Id: If82ec667d0a5c8531ce83868f88088a7afd166b2
Reviewed-by: Alex <3f9b327efd0f2b2e684d51660e829d1496ae5581@nokia.com>

kobolabs/qtlocation,amccarthy/qtlocation,kobolabs/qtlocation,amccarthy/qtlocation,kobolabs/qtlocation,amccarthy/qtlocation