hostdb cores in reply_to_cont() when running out of space

In HostDBContinuation::lookup_done() if we can't allocate memory in the cache we will free some space (delete_block())
and then return a NULL. Also, the error message "out of room in hostdb for reverse DNS data" gets printed
out.

This NULL pointer will be dereferenced later when we call reply_to_cont().

bz 3191464