audio/avrcp: Fix crash when connecting to controllers

Patch 70fdb77d0af137aa859f267da976f610cd9bbbd2 has actually caused the
following regression since no player is set:

Invalid read of size 8
  at 0x435B8E: avrcp_player_value_rsp (avrcp.c:2150)
  by 0x42FB83: control_response (avctp.c:831)
  by 0x42FB83: session_cb (avctp.c:996)
  by 0x50C8E39: g_main_context_dispatch (in /usr/lib64/libglib-2.0.so.0.4600.2)
  by 0x50C91CF: ??? (in /usr/lib64/libglib-2.0.so.0.4600.2)
  by 0x50C94F1: g_main_loop_run (in /usr/lib64/libglib-2.0.so.0.4600.2)
  by 0x40C258: main (main.c:687)
Address 0x58 is not stack'd, malloc'd or (recently) free'd
1 file changed