matroskadec: Fix a double negation typo

This typo has existed since this code was added in c16582579.
Newer versions of clang pointed out that this comparison always
was true (since the result of the negation is either 0 or 1, while
AVDISCARD_ALL has the value 48).

Signed-off-by: Martin Storsjö <martin@martin.st>
Merge commit '98569d89638efc325042bd0081e9a02181afdafa'

* commit '98569d89638efc325042bd0081e9a02181afdafa':
  matroskadec: Fix a double negation typo

See: 2c5b92fe90463402be6bf611ef40195f86f434fd
Merged-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/on2avc: Fix out of array access

Fixes CID1206648

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