mpegaudiodec: fix short_start calculation

The value should be always 3, as it follows from the specification.

Fix a stack buffer overflow in exponents_from_scale_factors as reported
by asan. Thanks to Dale Curtis for the sample vector.

libavcodec/mpegaudiodec.c

@@ -211,7 +211,7 @@ static void ff_compute_band_indexes(MPADecodeContext *s, GranuleDef *g)
             else
                 g->long_end = 6;
 
-            g->short_start = 2 + (s->sample_rate_index != 8);
+            g->short_start = 3;
         } else {
             g->long_end    = 0;
             g->short_start = 0;