AudioEncoderIsac: Print error code if CHECK for successful encoding fails

This will hopefully make the crash in bug 4577 easier to understand if
it happens again.

BUG=4577
R=henrik.lundin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/52389004

Cr-Commit-Position: refs/heads/master@{#9100}

webrtc/modules/audio_coding/codecs/isac/audio_encoder_isac_t_impl.h

@@ -145,8 +145,9 @@ AudioEncoder::EncodedInfo AudioEncoderDecoderIsacT<T>::EncodeInternal(
   {
     CriticalSectionScoped cs(state_lock_.get());
     r = T::Encode(isac_state_, audio, encoded);
+    CHECK_GE(r, 0) << "Encode failed (error code "
+                   << T::GetErrorCode(isac_state_) << ")";
   }
-  CHECK_GE(r, 0);
 
   // T::Encode doesn't allow us to tell it the size of the output
   // buffer. All we can do is check for an overrun after the fact.