Bluetooth: LE: BAP: Check buf len before using it

Check whether the length of buf is consistent with the valid data
received for op code BT_BAP_BASS_OP_ADD_SRC and BT_BAP_BASS_OP_MOD_SRC.

If the length of buf is inconsistent with the valid data received, the
response is error code BT_ATT_ERR_WRITE_REQ_REJECTED instead of other
errors.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>