Revert "avprobe: Zero the allocated avio buffer memory"

This reverts commit 0e0538ae.

The valgrind warning was a false positive due to OSX implementation of
printf (invoking a strnlen), while this code is actually fine, since the
format specifier %.*s guarantes that no more than buf_size bytes from
buf will be read.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>