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.

src/os_mswin.c

@@ -201,6 +201,8 @@ int _stricoll(char *a, char *b)
     void
 mch_exit(int r)
 {
+    exiting = TRUE;
+
     display_errors();
 
     ml_close_all(TRUE);		/* remove all memfiles */