Fix for infinite loop and memory leak in reaction code.

If the TU % cost of the weapon action is low enough, the calculated cost get to 0, so the unit will always be able to react.

Fixes http://openxcom.org/bugs/openxcom/issues/818 #818
Merge pull request #951 from Smjert/ReactionTUFix

Fix for infinite loop and memory leak in reaction code.