If -U is specified, flush the file after creating it, so it's not zero-length.

If -U is specified, flush the file after creating it, so it's not zero-length.

Reading from a capture file that has not yet received any packets fails
with "truncated dump file"; to avoid this, flush the file (forcing the
pcap header out) immediately after opening it.

(Added Romain to the credits.)

Reviewed-By: Guy Harris <[email protected]>