[bug2018 indirectly] Trap in mixerGetLineControlA

[Ann and Jason Edmeades]

I was trying to reproduce a problem in a game to see if it still existed but
couldn't actually run it. Debugging showed our Unification caused a trap
because we tried to allocate a massive block, failed (silently), left a null
pointer which was subsequently dereference. 

The reason is in mixerGetLineControlsA we are passed an ASCII structure +
what to do. The MSDN documents the cControls field MUST be populated to the
number of controls, and we currently multiply that by the size of a
structure and try to allocate it. However this game doesn't fill it in, and
the value is a residual value off the stack.

Debugging windows shows it *specifically* looks for the case where the parm
is 2 (ie MIXER_GETLINECONTROLSSF_ONEBYTYPE) and if it is that, allocates 1x
sizeof(structure). I have modified our code to do the same. I couldn't think
of a testcase which could prove precisely this was the case so here's the
patch.

Also, game tested 'thoroughly' under wine to confirm the actual bug didn't
appear for me either :-))

Changelog

Avoid trap in mixerGetLineControlsA when cControls is uninitialized and
MIXER_GETLINECONTROLSSF_ONEBYTYPE requested.

Jason
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