Fix null pointer access in handle_rc_ctrl_features

There is a chance the callback gets invoked right after a AVRC
disconnection and bt_rc_ctrl_callbacks has been cleared.

Tag: stability
Bug: 242208896
Test: presubmit, Bluetooth calling test items
(cherry picked from https://android-review.googlesource.com/q/commit:722854df05ddad0567f5c30db2491afc90d15228)
Merged-In: I649ac336022a20894d2311313d7ed68687bc70a3
Change-Id: I649ac336022a20894d2311313d7ed68687bc70a3
Bug: 263323082