number if strerror_r() did not succeed for whatever reason.

This avoids the need for the application to fill the string in case
strerror_r() fails, for example because the error code is not known.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@23015 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b