lavc/ass_split: check for NULL pointer in ff_ass_split_override_codes().

This is consistent with the other ff_ass_split_* functions.

It also fixes a crash when trying to split a dialog with text=NULL
(which seems to happen when the text of the dialog is empty); basically,
this commit fixes crashes when trying to encode an empty text subtitle
dialog (see subrip and mov_text encoders).

Fixes Ticket2048.
(cherry picked from commit c83002a4f8042ccfa0688a9a18e8fa0369c1fda8)
tiffdec: Use the correct height field.

Fixes Ticket913

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 4784a135b2b0fe4d1b4c6256bd37265fc45aed3d)

Conflicts:
	libavcodec/tiff.c
(cherry picked from commit fe0e64ca6431c2f606bc702c1a4e230f22531a4f)