stb_vorbis: Remove spurious assignment to val

This is definitely unnecessary, or at least I can't find anything
in the Vorbis spec that would indicate anything special happening
here.

Fixes issue #816.

stb_vorbis.c

@@ -3872,8 +3872,7 @@ static int start_decoder(vorb *f)
                unsigned int div=1;
                for (k=0; k < c->dimensions; ++k) {
                   int off = (z / div) % c->lookup_values;
-                  float val = mults[off];
-                  val = mults[off]*c->delta_value + c->minimum_value + last;
+                  float val = mults[off]*c->delta_value + c->minimum_value + last;
                   c->multiplicands[j*c->dimensions + k] = val;
                   if (c->sequence_p)
                      last = val;