prevent double-free in mapnik::projection
prevent double-free in mapnik::projection

A double-free can happen when init_proj4 is called outside of the
constructor (for projections with defer_proj_init=true) and when that
call fails. In this case proj_ctx_ is not set to NULL and then freed
again in the destructor. Set to null to avoid second pj_ctx_free call.

See #2170

mapnik/mapnik,zerebubuth/mapnik,tomhughes/mapnik,whuaegeanse/mapnik,mbrukman/mapnik,qianwenming/mapnik,pnorman/mapnik,yohanboniface/python-mapnik,mapycz/python-mapnik,cjmayo/mapnik,sebastic/python-mapnik,stefanklug/mapnik,davenquinn/python-mapnik,CartoDB/mapnik,rouault/mapnik,tomhughes/python-mapnik,garnertb/python-mapnik,Uli1/mapnik,manz/python-mapnik,Airphrame/mapnik,mapnik/python-mapnik,Airphrame/mapnik,tomhughes/mapnik,mapnik/mapnik,mapnik/mapnik,tomhughes/python-mapnik,mapycz/mapnik,davenquinn/python-mapnik,jwomeara/mapnik,cjmayo/mapnik,naturalatlas/mapnik,mapycz/python-mapnik,pramsey/mapnik,qianwenming/mapnik,rouault/mapnik,mapycz/mapnik,qianwenming/mapnik,Airphrame/mapnik,whuaegeanse/mapnik,qianwenming/mapnik,stefanklug/mapnik,lightmare/mapnik,whuaegeanse/mapnik,sebastic/python-mapnik,zerebubuth/mapnik,Uli1/mapnik,jwomeara/mapnik,manz/python-mapnik,jwomeara/mapnik,tomhughes/mapnik,naturalatlas/mapnik,pnorman/mapnik,tomhughes/python-mapnik,cjmayo/mapnik,tomhughes/mapnik,rouault/mapnik,CartoDB/mapnik,mapycz/mapnik,garnertb/python-mapnik,jwomeara/mapnik,yohanboniface/python-mapnik,yohanboniface/python-mapnik,mbrukman/mapnik,zerebubuth/mapnik,lightmare/mapnik,naturalatlas/mapnik,manz/python-mapnik,mbrukman/mapnik,lightmare/mapnik,Airphrame/mapnik,rouault/mapnik,mapnik/python-mapnik,stefanklug/mapnik,Uli1/mapnik,cjmayo/mapnik,pnorman/mapnik,qianwenming/mapnik,mapnik/python-mapnik,CartoDB/mapnik,sebastic/python-mapnik,mapnik/mapnik,pramsey/mapnik,davenquinn/python-mapnik,Uli1/mapnik,garnertb/python-mapnik,lightmare/mapnik,naturalatlas/mapnik,pramsey/mapnik,pnorman/mapnik,pramsey/mapnik,mbrukman/mapnik,whuaegeanse/mapnik,stefanklug/mapnik