audio_route: Fix cleanup of 'audio_route' structure

'path_free' function was not updating 'num_mixer_paths' after
freeing 'mixer_path' array, which was leading to a crash during
a subsequent call to 'path_get_by_name'.

Bug: 35445650
Change-Id: Ie47bb2e548fde89d9519ac726618fcf2e72da286
Test: with BT A2DP connected, kill system_server, watch logcat
1 file changed