w64dec: fix skipping of unknown guids

Regression since 14d50c1.
Fixes #2932.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
w64dec: fix end position of summarylist guid

Noticed-by: James Almer

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avutil/fifo: assert that theres enough data in the fifo on drain calls.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>