lavfi/anullsrc: fix format negotiation

Samplerate and channel layouts must be set in query_formats.
lavfi/anullsrc: simplify logging code in config_props
lavfi/anullsrc: remove extra parentheses
lavfi/anullsrc: add missing checks
h264: dont report rows as finished after a missing slice

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