netgraph/ng_bridge: Handle send errors during loop handling

If sending out a packet fails during the loop over all links, the
allocated memory is leaked and not all links receive a copy. This
patch fixes those problems, clarifies a premature abort of the loop,
and fixes a minory style(9) bug.

PR: 255430
Submitted by: Dancho Penev
Tested by: Dancho Penev
Differential Revision: https://reviews.freebsd.org/D30008

(cherry picked from commit a56e5ad6903037861457da754574b4903dcb0e7e)