ssa.c: In ssa_upstream_update_conn, when in SSA_DB_FIELD_DEFS phase, only store rbuf into SSA DB when not the same as rhdr

This handles NULL field defs as is used for variable size tables such
as the PKEY table in SMDB.

This was causing double deletion of rhdr.

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