patch 8.1.0990: floating point exception with "%= 0" and "/= 0"

Problem:    Floating point exception with "%= 0" and "/= 0".
Solution:   Avoid dividing by zero. (Dominique Pelle, closes #4058)
diff --git a/src/version.c b/src/version.c
index 1396574..f9fea1c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -780,6 +780,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    990,
+/**/
     989,
 /**/
     988,