LibAudio: Skip empty MP3 scale factor bands in stereo intensity process

These were intentionally set up to be at the end of the granule size,
but since the stereo intensity loop is intentionally using a <= end
comparison (that’s how the scale factor bands work), we must skip these
dummy bands which would otherwise cause an out-of-bounds index.

Userland/Libraries/LibAudio/MP3Loader.cpp

@@ -724,6 +724,9 @@ void MP3LoaderPlugin::process_stereo(MP3::MP3Frame& frame, size_t granule_index)
 
     auto process_intensity_stereo = [&](MP3::Tables::ScaleFactorBand const& band, float intensity_stereo_ratio) {
         for (size_t i = band.start; i <= band.end; i++) {
+            // Superflous empty scale factor band.
+            if (i >= MP3::granule_size)
+                continue;
             float const sample_left = granule_left.samples[i];
             float const coeff_l = intensity_stereo_ratio / (1 + intensity_stereo_ratio);
             float const coeff_r = 1 / (1 + intensity_stereo_ratio);