Fixed Grizzly crash (issue #244), see description

When Grizzly tried to fire while its fire point was inside enemy unit convol, the Spring engine was crashing.
Easily reproduced if you spawned the Grizzly in water and a ball of 50 enemy Halberds above it.
Apparently 0 range is causing some div-by-zero crash in the engine. I seached and no other unit script tries to set weapon range to 0.

This fixes the crash, but I didn't look at Grizzly behaviour regarding shooting inside allied units as mentioned in issue #244.
This is because I don't understand the purpose of the original code added in:
https://github.com/ZeroK-RTS/Zero-K/commit/5f0424a7e55fc69af58b86d710d5c0e4a108830e

/Rafal