Awayviewmobj sanity checks

This fixes the P_CameraCheckWater crash encountered in Acidic Alpines in Kimokawaiii, and possibly a few similar scenarios. Turns out that Acidic Alpines uses a MT_THOK awayviewmobj for some reason, which eventually is removed from the map (leading to the crash).

The problem is really just that the game never really considered an alt view cam mobj that gets removed when in use. =P