nx-match: Succeed pulling 0-byte nx-match from NULL buffer.

I don't think this corner case can come up in a real OpenFlow message,
because the presence of the OpenFlow header guarantees that the ofpbuf's
data is nonnull, but it did in a simple test that is coming up in a few
commits.

Signed-off-by: Ben Pfaff <blp@nicira.com>