ass_face_stream: don't leak first struct if second alloc fails
ass_face_stream: don't leak first struct if second alloc fails

Bug in commit a7f67df5f96f03fab6661d90d716c16e0fab4a21, which introduced
the alloc check. Before that, alloc failure would result in illegal
memory access, so at least this was an improvement.

Fixes CID 350862 found by Coverity Scan.

MrSmile/libass,ShiftMediaProject/libass,astiob/libass,xbmc/libass,MrSmile/libass,libass/libass,astiob/libass,xbmc/libass,libass/libass,ShiftMediaProject/libass