make AAS_LoadAASFile work with zero-sized lumps

AAS_LoadAASLump will return 0/NULL in an error case. However it will also
return the input buffer pointer if length is 0, and the input buffer pointer
may be 0/NULL in the case where AAS_LoadAASLump is meant to allocate memory.

This means that it can return 0/NULL in a perfectly legal case where the lump
length is 0. Since AAS_LoadAASFile inteprets this as a fatal error, that's
bad.

I'm not sure how to best disambiguate this, but a low-touch fix is for the
caller of AAS_LoadAASLump to be OK with a returned NULL pointer if the lump
length is 0.

Also: added a missing break statement to avoid a spurious "don't know what to
do" message when using the aasinfo switch.