Fix FS_FOpenFileReadDir non-zero file handle when file not found in pk3

If a pk3 search path is passed to FS_FOpenFileReadDir, a non-zero
file handle is returned if file is not found. This causes incorrect
behavior in FS_ReadFileDir (when a pk3 search path is passed in)
which only checks file handle, not length, for seeing if file exists.

I don't know of any issues in ioq3 caused by this.