Fixed Aec handle index in EchoCancellationImpl

that was not updated as it should.

The bug caused no negative impact at all apart
from a missed check that the Aec handles above
index 0 were not null. That check is, however,
done elsewhere so there was no negative impact
of this bug.

BUG=

Review URL: https://codereview.webrtc.org/1716203002

Cr-Commit-Position: refs/heads/master@{#11863}

webrtc/modules/audio_processing/echo_cancellation_impl.cc

@@ -116,6 +116,8 @@ int EchoCancellationImpl::ProcessRenderAudio(const AudioBuffer* audio) {
                                   audio->split_bands_const_f(j)[kBand0To8kHz],
                                   (audio->split_bands_const_f(j)[kBand0To8kHz] +
                                    audio->num_frames_per_band()));
+
+      handle_index++;
     }
   }