patch 8.0.0307: asan detects a memory error when EXITFREE is defined

Problem:    Asan detects a memory error when EXITFREE is defined. (Dominique
            Pelle)
Solution:   In getvcol() check for ml_get_buf() returning an empty string.
            Also skip adjusting the scroll position.  Set "exiting" in
            mch_exit() for all systems.
diff --git a/src/window.c b/src/window.c
index ee5f7aa..6b7bd35 100644
--- a/src/window.c
+++ b/src/window.c
@@ -5708,7 +5708,10 @@
     wp->w_height = height;
     wp->w_skipcol = 0;
 
-    scroll_to_fraction(wp, prev_height);
+    /* There is no point in adjusting the scroll position when exiting.  Some
+     * values might be invalid. */
+    if (!exiting)
+	scroll_to_fraction(wp, prev_height);
 }
 
     void