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_amiga.c

@@ -889,6 +889,8 @@ mch_early_init(void)
     void
 mch_exit(int r)
 {
+    exiting = TRUE;
+
     if (raw_in)			    /* put terminal in 'normal' mode */
     {
 	settmode(TMODE_COOK);