patch 8.0.0463: side effects from resetting 'compatible' in defaults.vim

Problem:    Resetting 'compatible' in defaults.vim has unexpected side
            effects. (David Fishburn)
Solution:   Only reset 'compatible' if it was set.
diff --git a/src/version.c b/src/version.c
index ff1c9b8..c1ac29d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    463,
+/**/
     462,
 /**/
     461,