patch 8.0.0636: when reading the undo file fails may use uninitialized data

Problem:    When reading the undo file fails may use uninitialized data.
Solution:   Always clear the buffer on failure.

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    636,
 /**/
     635,
 /**/