Fix some 16-bit int issues in the multistream API.

With 120 ms frames and 6 or more channels, the total number of
 samples in the buffer could overflow.
diff --git a/src/opus_multistream_encoder.c b/src/opus_multistream_encoder.c
index 66c75ca..1847bd5 100644
--- a/src/opus_multistream_encoder.c
+++ b/src/opus_multistream_encoder.c
@@ -356,7 +356,7 @@
 )
 {
    const float *float_src;
-   int i;
+   opus_int32 i;
    float_src = (const float *)src;
    for (i=0;i<frame_size;i++)
 #if defined(FIXED_POINT)
@@ -377,7 +377,7 @@
 )
 {
    const opus_int16 *short_src;
-   int i;
+   opus_int32 i;
    short_src = (const opus_int16 *)src;
    for (i=0;i<frame_size;i++)
 #if defined(FIXED_POINT)