[opus] Possible pointer math issue in 1.1

[O'Connor, Kevin]

While integrating 1.1, I ran into some errors with my compiler and, after examining the code, saw what
might be some invalid pointer math in opus_encoder.c. In optimize_framesize(), the first parameter 'x' is
typed as opus_val16 and pointer math is done on the variable (in the 'if (buffering)' block) and then x is
passed to the downmix function. I see that the downmix function uses the proper type, but it
appears to me that the pointer adjustment performed in that 'if (buffering)' block is always using 16-bit.

Kevin O'Connor