patch 9.2.0039: potential integer underflow in screen_line()

Problem:  In screen_line(), there is a potential integer underflow when
          accessing ScreenAttrs[off_to - 1] if off_to is zero.
          (Coverity CID 1681430, after v9.2.0017)
Solution: Add a check to ensure off_to > 0 before accessing the
          previous attribute index.

related: #19272
closes:  #19479

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/screen.c

@@ -848,7 +848,7 @@ skip_opacity:
 #ifdef FEAT_GUI
 		&& !gui.in_use
 #endif
-		&& col + coloff > 0)
+		&& col + coloff > 0 && off_to > 0)
 	{
 	    if (ScreenAttrs[off_to] == ScreenAttrs[off_to - 1])
 	    {

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    39,
 /**/
     38,
 /**/