Change PIT_CheckThing() to perform a MF2_PASSMOBJ check on both things before doing a test against the z-axis.

This allows us to disable MF2_PASSMOBJ on the player corpse while checking for a spawn point in G_CheckSpot(), fixing bug 925.

git-svn-id: http://odamex.net/svn/root/trunk@3668 b0bcef46-2027-0410-bd56-ab726e9a1d04