dsp: q6core: validate payload size before access for AVCS

Payload size is not checked before payload access for AVCS.
Check size to avoid out-of-boundary memory access.

Change-Id: I6de3342617bd4f3fb8849ad2230dd57c07469372
Signed-off-by: Shalini Manjunatha <quic_c_shalma@quicinc.com>