patch 7.4.1837

Problem:    The BufUnload event is triggered twice, when :bunload is used with
             set to  or .
Solution:   Do not trigger the event when ml_mfp is NULL. (Hirohito Higashi)

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1837,
 /**/
     1836,
 /**/