echo_frame should be initialized as frame

[matlab2000]

there's no check for apm->ProcessReverseStream(echo_frame); actually,it will return
kBadSampleRateError = -7.
just treat the echo_frame like frame,initialize it and the prog will run happily. the aec module will work.
webrtc::AudioFrame *echo_frame = NULL;
if (is_echo_cancel) {
echo_frame = new webrtc::AudioFrame();
echo_frame->num_channels_ = 1;
echo_frame->sample_rate_hz_ = 16000;
echo_frame->samples_per_channel_ = frame->sample_rate_hz_ * frame_step / 1000.0;
}