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/os_win32.c b/src/os_win32.c
index d35061f..adbb199 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -2538,8 +2538,9 @@
     void
 mch_exit(int r)
 {
-    stoptermcap();
+    exiting = TRUE;
 
+    stoptermcap();
     if (g_fWindInitCalled)
 	settmode(TMODE_COOK);