audio/avrcp: Fix crash if no player is available

It seems some Samsung Android devices may actually report no players
at all causing the following crash:

Program terminated with signal 11, Segmentation fault.
  #0  set_ct_player (player=0x0, session=<value optimized out>)
    at profiles/audio/avrcp.c:3139
  #1  0xb76c0aab in player_remove (data=0xb849a100)
    at profiles/audio/avrcp.c:3278
1 file changed