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Avoid a potential divide-by-zero.

Reported by clang's static analysis in
 https://github.com/xiph/opusfile/issues/16
This would have taken about 22 TB of junk data before the first
 decoded sample to trigger, so not very likely to occur in
 practice.

Thanks to kcgen for filing the report.

--- a/src/opusfile.c

+++ b/src/opusfile.c

@@ -1782,6 +1782,7 @@

 /*Compute an average bitrate given a byte and sample count.

   Return: The bitrate in bits per second.*/

 static opus_int32 op_calc_bitrate(opus_int64 _bytes,ogg_int64_t _samples){

+  if(OP_UNLIKELY(_samples<=0))return OP_INT32_MAX;

   /*These rates are absurd, but let's handle them anyway.*/

   if(OP_UNLIKELY(_bytes>(OP_INT64_MAX-(_samples>>1))/(48000*8))){

     ogg_int64_t den;

@@ -1791,7 +1792,6 @@

     den=_samples/(48000*8);

     return (opus_int32)((_bytes+(den>>1))/den);

   }

-  if(OP_UNLIKELY(_samples<=0))return OP_INT32_MAX;

   /*This can't actually overflow in normal operation: even with a pre-skip of

      545 2.5 ms frames with 8 streams running at 1282*8+1 bytes per packet

      (1275 byte frames + Opus framing overhead + Ogg lacing values), that all