Added check for valid wall_atomtype in .mdp

Otherwise, mdrun would dump core when the garbage got dereferenced.

Note that this does not need to be merged into release-4-6, because
c7a82654 already fixes it.

Fixes #700

Change-Id: I39f440595f7b112d4f01b5e32218ec45a2a97370
Merge "Added check for valid wall_atomtype in .mdp" into release-4-5-patches