patch 8.0.0642: writefile() continues after detecting an error

Problem:    writefile() continues after detecting an error.
Solution:   Bail out as soon as an error is detected. (suggestions by Nikolai
            Pavlov, closes #1476)
diff --git a/src/version.c b/src/version.c
index 365dbc5..6f5c893 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    642,
+/**/
     641,
 /**/
     640,