nx-match: Correct writing of value and length in set_field_to_ofast()

nx-match: Correct writing of value and length in set_field_to_ofast()

ofpbuf_put_* may reallocate the underlying buffer of the ofpbuf and
thus writing data after a ofpbuf_put_* call must write to memory
relative to the pointer returned by the call.

Prior to this change the length and trailing value would not be written to
the set_field action if ofpbuf_put_* may reallocated the underlying buffer.

Also make use of ofpbuf_put_zero() to avoid calling memset() directly.

Tested-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>