following up from r10000 and r10004, fix the hud squad message display for support ship aborts
(and in the process optimize things slightly)

git-svn-id: svn://svn.icculus.org/fs2open@10406 387891d4-d844-0410-90c0-e4c51a9137d3
Mantis #3004: ship_get_ship_with_dock_bay was written in such a way that it allowed a negative array index to create all sorts of havoc

git-svn-id: svn://svn.icculus.org/fs2open@10407 387891d4-d844-0410-90c0-e4c51a9137d3