This makes rdt work again, which has been broken since
603b8bc2a109978c8499b06d2556f1433306eca7. This commit made
opening a demuxer without a file (or in this case, with a filename
which can't be opened) fail, unless the demuxer actually declared
AVFMT_NOFILE.
Signed-off-by: Martin Storsjö <martin@martin.st>