patch 8.1.2071: when 'wincolor' is set text property changes highlighting

Problem:    When 'wincolor' is set text property changes highlighting. (Andy
            Stewart)
Solution:   Fix combining colors. (closes #4968)
diff --git a/src/drawline.c b/src/drawline.c
index 5916458..43604c8 100644
--- a/src/drawline.c
+++ b/src/drawline.c
@@ -1367,6 +1367,7 @@
 
 		text_prop_attr = 0;
 		text_prop_combine = FALSE;
+		text_prop_type = NULL;
 		if (text_props_active > 0)
 		{
 		    // Sort the properties on priority and/or starting last.
@@ -1406,8 +1407,11 @@
 # ifdef FEAT_TEXT_PROP
 	    else if (text_prop_type != NULL)
 	    {
-		char_attr = hl_combine_attr(
-			line_attr != 0 ? line_attr : win_attr, text_prop_attr);
+		char_attr = hl_combine_attr(line_attr != 0
+						? line_attr
+						: syntax_attr != 0
+						    ? syntax_attr
+						    : win_attr, text_prop_attr);
 	    }
 # endif
 	    else if (line_attr != 0 && ((fromcol == -10 && tocol == MAXCOL)