Heap out-of-bounds read in dr_flac SEEKTABLE metadata parsing

A crafted FLAC file opened with drflac_open_*_with_metadata() causes a heap-based out-of-bounds read when the SEEKTABLE metadata block is parsed.

drflac__read_and_decode_metadata allocates the seekpoint buffer as seekpointCount * sizeof(drflac_seekpoint) (24 bytes per point, padded) but sets metadata.rawDataSize to the on-disk block size (DRFLAC_SEEKPOINT_SIZE_IN_BYTES is 18). A consumer that reads pRawData for the documented rawDataSize bytes therefore reads past the end of the allocation. SEEKTABLE is the only metadata handler where the advertised size and the allocation disagree.

PoC: a 77-byte FLAC with a 35-byte SEEKTABLE block → 35 / 18 = 1 seekpoint (24-byte buffer) but rawDataSize = 35, an 11-byte over-read, reproducible every run under ASan. Fix is one line: set rawDataSize = seekpointCount * sizeof(drflac_seekpoint).