Add missing AVRCP message length checks inside avrc_msg_cback

Explicitly check the length of the received message before
accessing the data.

Bug: 111803925
Bug: 79883824
Test: POC scripts
Change-Id: I00b1c6bd6dd7e18ac2c469ef2032c7ff10dcaecb
Merged-In: I00b1c6bd6dd7e18ac2c469ef2032c7ff10dcaecb
(cherry picked from commit 282deb3e27407aaa88b8ddbdbd7bb7d56ddc635f)