LibAudio: Fix UTF-8 decoding logic in FLAC decoding :^)

The problem here was that the multi-byte UTF-8 encoded characters
were taking one byte too much, misaligning the data completely
and eventually crashing the program on the 128th frame.

This change reduces the for loop by one, as it has been already
calculated from the start_byte variable.

Userland/Libraries/LibAudio/FlacLoader.cpp

@@ -802,7 +802,7 @@ u64 read_utf8_char(InputStream& input)
     u8 bits_from_start_byte = 8 - (length + 1);
     u8 start_byte_bitmask = AK::exp2(bits_from_start_byte) - 1;
     character = start_byte_bitmask & start_byte;
-    for (u8 i = length; i > 0; --i) {
+    for (u8 i = length - 1; i > 0; --i) {
         input.read(single_byte_buffer);
         u8 current_byte = single_byte_buffer[0];
         character = (character << 6) | (current_byte & 0b00111111);