If the format probe hits end of file, do not add the error code
to the buffer position.  This is obviously wrong, and with a
small input file would cause a negative buffer overflow.

Fixes issue 1818.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22571 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b