[josm_elevationprofile] Bugfix: Cursor showed always zero elevation when no SRTM values are present
[josm_elevationprofile] Bugfix: Cancel grid rendering, if no SRTM values are present

git-svn-id: http://svn.openstreetmap.org/applications/editors/josm/plugins@29979 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4