Regression - Uninitialized geiger counter volume variable assertion on Debug builds

[JoelTroch]

Back in 2014, PR #1539 was merged to fix several assertion issues on Debug builds about uninitialized variables.

However, the first HL25 update commit (0fc8913#diff-9203423ac932f9aa4e788c44e510c924891af4ba2f6453abdf028a24add6d412) seems to have reverted that change in the geiger (cl_dll/geiger.cpp) volume (flVol variable in CHudGeiger::Draw(float flTime)) thus bringing back that assertion issue.

This can be easily fixed by reverting the (accidental ?) change from the mentioned HL25 update commit and making sure the concerned code works like the already merged PR.