odp-util: Add checking to prevent buffer overflow when parsing push_nsh

odp-util: Add checking to prevent buffer overflow when parsing push_nsh

Previously, the buffer size of 'struct ofpbuf b' is less than the
size of 'char buf[512]', this could cause memory overflow of ofpbuf
when calling ofpbuf_put_hex. This patch fixes it.

Reported-at: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=10865
Signed-off-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>