stb_vorbis: A few UB fixes.

Fixes issue #1018.

stb_vorbis.c

@@ -1055,7 +1055,7 @@ static float float32_unpack(uint32 x)
    uint32 sign = x & 0x80000000;
    uint32 exp = (x & 0x7fe00000) >> 21;
    double res = sign ? -(double)mantissa : (double)mantissa;
-   return (float) ldexp((float)res, exp-788);
+   return (float) ldexp((float)res, (int)exp-788);
 }
 
 
@@ -3379,7 +3379,7 @@ static int vorbis_decode_packet_rest(vorb *f, int *len, Mode *m, int left_start,
       // this isn't to spec, but spec would require us to read ahead
       // and decode the size of all current frames--could be done,
       // but presumably it's not a commonly used feature
-      f->current_loc = -n2; // start of first frame is positioned for discard
+      f->current_loc = 0u - n2; // start of first frame is positioned for discard (NB this is an intentional unsigned overflow/wrap-around)
       // we might have to discard samples "from" the next frame too,
       // if we're lapping a large block then a small at the start?
       f->discard_samples_deferred = n - right_end;
@@ -3963,7 +3963,7 @@ static int start_decoder(vorb *f)
                if (g->class_masterbooks[j] >= f->codebook_count) return error(f, VORBIS_invalid_setup);
             }
             for (k=0; k < 1 << g->class_subclasses[j]; ++k) {
-               g->subclass_books[j][k] = get_bits(f,8)-1;
+               g->subclass_books[j][k] = (int16)get_bits(f,8)-1;
                if (g->subclass_books[j][k] >= f->codebook_count) return error(f, VORBIS_invalid_setup);
             }
          }