odp-util: Fix a bug in parse_odp_push_nsh_action

odp-util: Fix a bug in parse_odp_push_nsh_action

In this piece of code, 'struct ofpbuf b' should always point to
metadata so that metadata can be filled with values through ofpbuf
operations, like ofpbuf_put_hex and ofpbuf_push_zeros. However,
ofpbuf_push_zeros may change the data pointer of 'struct ofpbuf b'
and therefore, metadata will not contain the expected values.

This patch fixes it by changing ofpbuf_push_zeros to
ofpbuf_put_zeros.

Reported-at: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=10863
Signed-off-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>