Remove a bogus check for a NULL buffer pointer.
Add a KASSERT that it is not NULL.

Found by:    Coverity Scan, CID 1009114
Reviewed by: kib
MFC after:   2 weeks
Null a pointer after it is freed so that when it is returned
the return value is NULL. Based on the returned flags, the
return value should never be inspected in the case where NULL
is returned, but it is good coding practice not to return a
pointer to freed memory.

Found by:    Coverity Scan, CID 1006096
Reviewed by: kib
MFC after:   2 weeks
Add missing 28th element to softdep types name array.

Found by:    Coverity Scan, CID 1007621
Reviewed by: kib
MFC after:   2 weeks
Add missing buffer releases (brelse) after bread calls that return
an error. One could argue that returning a buffer even when it is
not valid is incorrect, but bread has always returned a buffer
valid or not.

Reviewed by: kib
MFC after:   2 weeks
Implement beacon event debugging in athalq.