hastd: Fix crash on empty message

A HAST message can be empty, in which case ebuf_add_tail() does nothing
and ebuf_data() returns NULL because the size of the ebuf is zero, but
hast_proto_recv_hdr() asserts that the return value is not NULL,
resulting in an immediate crash if hastctl or hastd receive an empty
message.  This is trivially reproducable by running `hastctl status` or
`hastctl role init` (as the rc script does prior to stopping hastd).

To avoid this, don't try to grow the ebuf or receive additional data
if the header size is zero.

PR:		298085
MFC after:	3 days
Reviewed by:	kevans, gjb
Differential Revision:	https://reviews.freebsd.org/D59306

0 files changed, 0 insertions, 0 deletions