libmix: when terminate VA, reset mSignalBufferPre[] even va is not initialized

BZ: 191657

when terminate VA, reset mSignalBufferPre[] even va is not initialized
or else after output reconfiguration, unreseted mSignalBufferPre[] will cause
buffer renderdone flag mismatch and buffer status failure.

Change-Id: I8cffbbfc104882900718f7f85555f6cd6fc5a6b2
Signed-off-by: ywan171 <yi.a.wang@intel.com>
1 file changed