Crashing when using magnetopause and too long path to igrf.d.

[Erik Johansson]

magpack.c only accepts max 77 char (not 79 char) path to igrf.d and causes OVT to crash when using magnetopause.

This is a problem e.g. after installing OVT on Mac OS X in an extra subdirectory under /Applications, and OVT using the igrf.d in the installation directory (i.e. /Applications/<DIR>/Orbit Visualization Tool.app/Contents//java/app/mdata/igrf.d).

[Erik Johansson]

Added error message triggered before OVT (java code) passes on a too long path to magpack.c (C code). Code will still crash in the end but it will say what is wrong first.