patch 8.0.0503: endless loop in updating folds with 32 bit ints

Problem:    Endless loop in updating folds with 32 bit ints.
Solution:   Subtract from LHS instead of add to the RHS. (Matthew Malcomson)
diff --git a/src/version.c b/src/version.c
index 4b2ff6f..b6cc7d6 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    503,
+/**/
     502,
 /**/
     501,