input: fix vlc_attachment_New() error handling
input: fix vlc_attachment_New() error handling

Also use size_t for the size.

vlc-mirror/vlc,shyamalschandra/vlc,shyamalschandra/vlc,xkfz007/vlc,xkfz007/vlc,shyamalschandra/vlc,vlc-mirror/vlc,vlc-mirror/vlc,vlc-mirror/vlc,shyamalschandra/vlc,xkfz007/vlc,vlc-mirror/vlc,vlc-mirror/vlc,shyamalschandra/vlc,shyamalschandra/vlc,xkfz007/vlc,shyamalschandra/vlc,xkfz007/vlc,xkfz007/vlc,vlc-mirror/vlc,xkfz007/vlc