This is part of normal operation with some formats. A warning should
indicate that something is wrong, and the documentation for AV_LOG_WARNING
says: "Something somehow does not look correct."

Since this message is most likely useful for debugging only, raise the
log level accordingly.
Plus-one-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>