LibAudio: Don't overread MP3 granule samples if big_values is too large

There are at most 576 granule samples/frequency lines, but the side data
can specify that the big_values granule type take up to 1024 samples.
The spec says in 2.4.3.4.6 that we should always stop reading Huffman
data once we have 576 samples, so that is what this change does. I also
add some useful spec comments while I'm here.

Userland/Libraries/LibAudio/MP3Loader.cpp

@@ -521,7 +521,12 @@ MaybeLoaderError MP3LoaderPlugin::read_huffman_data(MP3::MP3Frame& frame, BigEnd
 
     size_t count = 0;
 
-    for (; count < granule.big_values * 2; count += 2) {
+    // 2.4.3.4.6: "Decoding is done until all Huffman code bits have been decoded
+    //             or until quantized values representing 576 frequency lines have been decoded,
+    //             whichever comes first."
+    auto max_count = min(granule.big_values * 2, MP3::granule_size);
+
+    for (; count < max_count; count += 2) {
         MP3::Tables::Huffman::HuffmanTreeXY const* tree = nullptr;
 
         if (count < region1_start) {
@@ -574,7 +579,7 @@ MaybeLoaderError MP3LoaderPlugin::read_huffman_data(MP3::MP3Frame& frame, BigEnd
     // count1 is not known. We have to read huffman encoded values
     // until we've exhausted the granule's bits. We know the size of
     // the granule from part2_3_length, which is the number of bits
-    // used for scaleactors and huffman data (in the granule).
+    // used for scalefactors and huffman data (in the granule).
     while (granule_bits_read < granule.part_2_3_length && count <= MP3::granule_size - 4) {
         auto const entry = MP3::Tables::Huffman::huffman_decode(reservoir, count1table, granule.part_2_3_length - granule_bits_read);
         granule_bits_read += entry.bits_read;
@@ -625,6 +630,8 @@ MaybeLoaderError MP3LoaderPlugin::read_huffman_data(MP3::MP3Frame& frame, BigEnd
         return LoaderError { LoaderError::Category::Format, m_loaded_samples, "Read too many bits from bit reservoir." };
     }
 
+    // 2.4.3.4.6: "If there are more Huffman code bits than necessary to decode 576 values
+    //             they are regarded as stuffing bits and discarded."
     for (size_t i = granule_bits_read; i < granule.part_2_3_length; i++) {
         LOADER_TRY(reservoir.read_bit());
     }