avcodec/error_resilience: factor er_supported() check out

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit afb18c55783362546b5e512ce01b7fe7bf5744d9)
avcodec/error_resilience: check that er is supported before attempting to read the status of the previous slice

Fixes incorrectly set error_occured and improves speed

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 90539cea336fd513c47295a03c164cb4a851166f)
avcodec/cabac: force get_cabac to be not inlined

works around bug in gccs inline asm register assignment
Fixes Ticket3177

gcc from 4.4 to 4.6 is affected at least, no non affected gccs known
clang seems not affected

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 0538b29ae8002c44f27bae8a1a6fc6e646998be5)