stream: always respect sector_size, fixes cdda://

Streams like CDDA have special requirements in what quantities data can
be read: you can only read a sector at once, not more and not less. The
stream_peek() function didn't respect that and set less (used internal
buffer size of 2048 bytes, instead of CD sector size of 2352 bytes), so
no data was read and EOF was accidentally set, making playback with
cdda:// fail.

This is a regression since commit 9a723f, but that commit merely exposed
the issue (the redundant seek would clear the EOF flag).