plugin/ssa_comparison.c: Bug fix in ssa_db_port_cmp

Wrong pointer was re-initialized in ssa_db_port_cmp()
method (p_tbl_rec_old was initialized twice instead
of p_tbl_rec_new).

The bug was found by Lenny's verification system:

==12273== Invalid read of size 8
==12273==    at 0xD358307: ssa_db_port_cmp (ssa_comparison.c:427)
==12273==    by 0xD358DD1: ssa_db_diff_var_size_table_cmp (ssa_comparison.c:667)
==12273==    by 0xD3594A9: ssa_db_diff_compare_subnet_tables (ssa_comparison.c:797)
==12273==    by 0xD35B3EA: ssa_db_compare (ssa_comparison.c:1376)
==12273==    by 0xD342219: core_extract_handler (core.c:777)
==12273==    by 0x340EA07850: start_thread (in /lib64/libpthread-2.12.so)
==12273==    by 0x340E2E890C: clone (in /lib64/libc-2.12.so)
==12273==  Address 0x12794b20 is not stack'd, malloc'd or (recently) free'd

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