[Proj] Fix reading failure of small GTX file.

[paul.chavent at fnac.net]

Hi.

As discussed in this thread (http://osgeo-org.1560.x6.nabble.com/How-to-offset-height-above-ellipsoid-td5195058.html), some GTX files fail to load as geoidgrids.

Indeed, as the smallest possible GTX file is compound of one tile (4 points), the resulting binary file may be smaller than the size of the header read by pj_gridinfo_init (160 bytes).

I've submitted a merge request on github (https://github.com/OSGeo/proj.4/pull/1) that should solve this issue Comments are welcome.

Regards.

Paul.