bomb launch interdiction: Fix crash on bombs missing target

bomb launch interdiction: Fix crash on bombs missing target

plane damage() prints @noisy with snprintf() even when it's null.  It
doesn't actually use the output then.  Some systems (GNU, Windows)
deal gracefully with printing null strings, others crash.  @noisy is
null when bombers or missiles miss and do collateral damage.  Affects
bomb, launch, and interdiction missions.  Broken in commit 820d755e5
"subs: Change pln_damage()'s parameter noisy to string prefix",
v4.3.33.

Fix by guarding the snprintf().

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>