patch 8.1.0921: terminal test sometimes fails; using memory after free

Problem:    Terminal test sometimes fails; using memory after free.
Solution:   Fee memory a bit later.  Add test to cover this.  Disable flaky
            screenshot test. (closes #3956)
diff --git a/src/terminal.c b/src/terminal.c
index f2f4ec0..e2ae2f6 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -4735,7 +4735,6 @@
 		    p2 += len2;
 		    /* TODO: handle different width */
 		}
-		vim_free(line1);
 
 		while (col < width)
 		{
@@ -4753,6 +4752,8 @@
 		    }
 		    ++col;
 		}
+
+		vim_free(line1);
 	    }
 	    if (add_empty_scrollback(term, &term->tl_default_color,
 						 term->tl_top_diff_rows) == OK)