Avoid problems when only one channel is silent

When one channel is silent and the other isn't, then mid and side are
not orthogonal, which can cause problems with the stereo code.
Now the side will always be zero in those cases.

Author: jmvalin

celt/bands.c

@@ -1243,6 +1243,11 @@ static unsigned quant_band(struct band_ctx *ctx, celt_norm *X,
    return cm;
 }
 
+#ifdef FIXED_POINT
+#define MIN_STEREO_ENERGY 2
+#else
+#define MIN_STEREO_ENERGY 1e-10f
+#endif
 
 /* This function is responsible for encoding and decoding a band for the stereo case. */
 static unsigned quant_band_stereo(struct band_ctx *ctx, celt_norm *X, celt_norm *Y,
@@ -1273,6 +1278,12 @@ static unsigned quant_band_stereo(struct band_ctx *ctx, celt_norm *X, celt_norm
 
    orig_fill = fill;
 
+   if (encode) {
+      if (ctx->bandE[ctx->i] < MIN_STEREO_ENERGY || ctx->bandE[ctx->m->nbEBands+ctx->i] < MIN_STEREO_ENERGY) {
+         if (ctx->bandE[ctx->i] > ctx->bandE[ctx->m->nbEBands+ctx->i]) OPUS_COPY(Y, X, N);
+         else OPUS_COPY(X, Y, N);
+      }
+   }
    compute_theta(ctx, &sctx, X, Y, N, &b, B, B, LM, 1, &fill);
    inv = sctx.inv;
    imid = sctx.imid;