avoid division by zero if there are still no spawn spots available after calling A_BrainAwake()
remove slime trails

mostly taken from Lee Killough's implementation in mbfsrc/P_SETUP.C:849-924,
with the exception that not the actual vertex coordinates are modified,
but pseudovertexes which are dummies that are *only* used in rendering,
i.e. r_bsp.c:R_AddLine()