[Xastir] Xastir crashing with TXRadar.geo or USRadar.geo

[Curt, WE7U]

Test conditions:  Xastir compiled with:

  > head summary.log
  AX25 Festival GPSMan GraphicsMagick libProj GeoTiff GDAL/OGR
  ShapeLib PCRE Dbfawk map_caching rtree libcurl

Enabled one of the two .geo's.  Zoomed in/out while it was still
loading the map from the internet.  Got a segfault one time and a
double-free fault another time.

It's likely there's a free somewhere in the GraphicsMagick or .geo
code that gets exercised due to the escape from a code block, in
turn caused by zooming.

--
Curt, WE7U: <www.eskimo.com/~archer/>     XASTIR: <www.xastir.org>
  "Lotto:  A tax on people who are bad at math." -- unknown
"Windows:  Microsoft's tax on computer illiterates." -- WE7U
The world DOES revolve around me:  I picked the coordinate system!