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
This patch needs to be ported to 2.3.x and master as well.