fix volume value preservation bug in NexuizDemoRecorder

Symptom: In the original code, the user's original volume value is not preserved (the default behavior is to mute the volume during fast forward) so the program fails to set the correct volume value back -- in fact it sets it to an illegal variable name.

Code problem: This bug happens in the case of user choosing to record the demo early on, i.e. the loop enters the second stage right away, so injectAtStart was never executed. Besides, srvLoop and firstLoop seem to do the same thing.

Fix: Make sure injectAtStart is executed exactly once.