There was a typo in the $fgbase/materials file where an alias was

There was a typo in the $fgbase/materials file where an alias was
pointing to a non-existant material name.  (This has been fixed.)

The alias code didn't check to see if this condition would occur so it
was putting a 'NULL' entry in the material library which was causing
the crash.  It now checks for and skips bad aliases and prints a
warning message.