patch 9.1.0069: ScreenLines may not be correctly initialized, causing hang

patch 9.1.0069: ScreenLines may not be correctly initialized, causing hang

Problem:  ScreenLines may not be correctly initialized, causing hang
          (Olaf Seibert, after 9.0.0220)
Solution: always initialize ScreneLines when allocating a screen
          (Olaf Seibert)

ScreenLines and related structures could be left uninitialized
causing a screen update to run into an infinite loop when using latin1
encoding.

Partly caused because by patch 9.0.0220, which makes mb_ptr2len return
zero for NUL

related: #12671
closes: #13946

Signed-off-by: Olaf Seibert <rhialto@falu.nl>
Signed-off-by: Christian Brabandt <cb@256bit.org>