flacdec: fix intermittent FLAC__STREAM_DECODER_ABORTED errors when seeking

flacdec: fix intermittent FLAC__STREAM_DECODER_ABORTED errors when seeking

When seeking in a local flac file (ie. operating pull-based), the decoder
would often just error out after the loop function sees a DECODER_ABORTED
status. This, however, is the read callback's way of telling our loop
function that pull_range failed and streaming should stop, in this case
because of the flush-start event that the seek handler pushed upstream
from the seeking thread. Handle this slightly better by storing the last
flow return from pull_range, so the loop function can evaluate it properly
when it encounters a DECODER_ABORTED and take the right action.

Fixes #578612.