error_concealment: Check that the reference is not NULL

In normal picture decoding this does not need to be checked but as
error concealment is run in the case of errors the availability of
references is less certain. This may be fixed differently at some
point so that all references are always filled in before the EC
code, in which case this should then be changed to an assert()

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
