ssa.c: Fix crash in downstream handler during sending SMDB

The crash was produced by the following scenario:

SSA tree: CORE + 1DL + 2AL

During SMDB update, one of the ALs was stuck in
the middle of SMDB receiving, and that prevented
the parent DL from receiving new SMDB updates
from CORE, but on each SSA_MSG_DB_UPDATE in DL
we had the following:
..
ssadb->p_db_field_tables = NULL;
ssadb->p_db_tables = NULL;
..
and after AL (that was stuck) kept running
DL crashed when continued sending corrupted SMDB.

Core dump BT:
*0 0x0000000000408564 in ssa_downstream_handle_query_field_defs (conn=0x7f57cc008590, hdr=0x7f57cc008870, events=1) at src/ssa.c:1890
*1 0x0000000000408ab0 in ssa_downstream_handle_op (conn=0x7f57cc008590, hdr=0x7f57cc008870, events=1, svc=0x7f57d00036f0, fds=0x7f57cc0008c0) at src/ssa.c:2001
*2 0x0000000000408ea3 in ssa_downstream_rrecv (conn=0x7f57cc008590, events=1, svc=0x7f57d00036f0, fds=0x7f57cc0008c0) at src/ssa.c:2080
*3 0x000000000040906e in ssa_downstream_handle_rsock_revents (conn=0x7f57cc008590, events=1, svc=0x7f57d00036f0, fds=0x7f57cc0008c0) at src/ssa.c:2125
*4 0x000000000040a728 in ssa_downstream_handler (context=0x7f57d00036f0) at src/ssa.c:2674
*5 0x00000038b9a07851 in start_thread () from /lib64/libpthread.so.0
*6 0x00000038b92e890d in clone () from /lib64/libc.so.6

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Sasha Kotchubievsky <sashakot@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>