nx-match: Fix use-after-free parsing matches.

nx-match: Fix use-after-free parsing matches.

Address pointed by header_ptr might be free'd due to realloc
happened in ofpbuf_put_hex(). Reported by valgrind in the test
379: check TCP flags expression in OXM and NXM.

Invalid write of size 4
    nx_match_from_string_raw (nx-match.c:1510)
    nx_match_from_string (nx-match.c:1538)
    ofctl_parse_nxm__ (ovs-ofctl.c:3325)
    ovs_cmdl_run_command (command-line.c:121)
    main (ovs-ofctl.c:137)

Address 0x7a2cc40 is 0 bytes inside a block of size 64 free'd
    free (vg_replace_malloc.c:530)
    ofpbuf_resize__ (ofpbuf.c:246)
    ofpbuf_put (ofpbuf.c:386)
    ofpbuf_put_hex (ofpbuf.c:414)
    nx_match_from_string_raw (nx-match.c:1488)
    nx_match_from_string (nx-match.c:1538)
    ofctl_parse_nxm__ (ovs-ofctl.c:3325)

Reported-by: William Tu <u9012063@gmail.com>
Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>