Fix harmless seqno mismatch in launch

Fix harmless seqno mismatch in launch

When an inefficient missile exploded on launch, it could damage
itself.  The damage had no effect, because the missile gets used up
right after.  But it triggers a seqno mismatch oops, in laun().  Fix
by making msl_launch() set PLN_LAUNCHED before the explosion.

This case was missed in commit 7bc63871, v4.3.14.  It didn't oops
until sequence numbers were added in v4.3.15.