Fix coverity warning.

- False positive about a shift with -1 when code_num overflows.

src/bitstream.c

@@ -61,16 +61,16 @@ void printf_bitstream(char *msg, ...)
 
 const bit_table_t *g_exp_table;
 
-//From wikipedia
-//http://en.wikipedia.org/wiki/Binary_logarithm#Algorithm
 static int floor_log2(unsigned int n) {
+  assert(n != 0);
+
   int pos = 0;
   if (n >= 1<<16) { n >>= 16; pos += 16; }
   if (n >= 1<< 8) { n >>=  8; pos +=  8; }
   if (n >= 1<< 4) { n >>=  4; pos +=  4; }
   if (n >= 1<< 2) { n >>=  2; pos +=  2; }
   if (n >= 1<< 1) {           pos +=  1; }
-  return ((n == 0) ? (-1) : pos);
+  return pos;
 }
 
 /**