ssa.c: Handle NULL table (used in SMDB for pkey table field definition)

Reported by Ilya when integrating core with access. core ibssa.log had:
ssa_downstream_handle_query_field_defs: pp_field_tbales index 5 (nil) len 0
ssa_downstream_send: ERROR - failed to allocate ssa_msg_hdr or sbuf of length 0 for response to op 2

To fix this, use END flag properly
In ssa_downstream_send, remove END flag setting and handle buf NULL and
len 0
In ssa_upstream_update_conn, for FIELD_DEFS and DATA, don't just check that
jsu header received but also END flag prior to moving to next phase of
database transfer.

Regressed with PRDB. Needs testing/verification with SMDB.

Signed-off by: Hal Rosenstock <hal@mellanox.com>