stb_vorbis: Fix handling of negative numbers in ilog.

For negative n, the original code went down the "n < (1<<4)"
path and performed an out-of-bounds array access. Fix the code
to agree with section 9.2.1 of the Vorbis spec. (Verified by
exhaustive testing of all 32-bit ints.)

Fixes issue #355.

stb_vorbis.c

@@ -986,17 +986,18 @@ static int ilog(int32 n)
 {
    static signed char log2_4[16] = { 0,1,2,2,3,3,3,3,4,4,4,4,4,4,4,4 };
 
+   if (n < 0) return 0; // signed n returns 0
+
    // 2 compares if n < 16, 3 compares otherwise (4 if signed or n > 1<<29)
    if (n < (1 << 14))
-        if (n < (1 <<  4))        return     0 + log2_4[n      ];
-        else if (n < (1 <<  9))      return  5 + log2_4[n >>  5];
+        if (n < (1 <<  4))            return  0 + log2_4[n      ];
+        else if (n < (1 <<  9))       return  5 + log2_4[n >>  5];
              else                     return 10 + log2_4[n >> 10];
    else if (n < (1 << 24))
-             if (n < (1 << 19))      return 15 + log2_4[n >> 15];
+             if (n < (1 << 19))       return 15 + log2_4[n >> 15];
              else                     return 20 + log2_4[n >> 20];
-        else if (n < (1 << 29))      return 25 + log2_4[n >> 25];
-             else if (n < (1 << 31)) return 30 + log2_4[n >> 30];
-                  else                return 0; // signed n returns 0
+        else if (n < (1 << 29))       return 25 + log2_4[n >> 25];
+             else                     return 30 + log2_4[n >> 30];
 }
 
 #ifndef M_PI