Sanitize input to opus_en/decoder_get_size.

src/opus_decoder.c

@@ -70,6 +70,8 @@ int opus_decoder_get_size(int channels)
 {
    int silkDecSizeBytes, celtDecSizeBytes;
    int ret;
+   if (channels<1 || channels > 2)
+      return 0;
    ret = silk_Get_Decoder_Size( &silkDecSizeBytes );
    if(ret)
       return 0;

src/opus_encoder.c

@@ -124,6 +124,8 @@ int opus_encoder_get_size(int channels)
 {
     int silkEncSizeBytes, celtEncSizeBytes;
     int ret;
+    if (channels<1 || channels > 2)
+        return 0;
     ret = silk_Get_Encoder_Size( &silkEncSizeBytes );
     if (ret)
         return 0;