patch 8.1.2105: MS-Windows: system() may crash

Problem:    MS-Windows: system() may crash.
Solution:   Do not use "itmp" when it is NULL. (Yasuhiro Matsumoto,
            closes #5005)
diff --git a/src/version.c b/src/version.c
index 29273b5..229312b 100644
--- a/src/version.c
+++ b/src/version.c
@@ -754,6 +754,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2105,
+/**/
     2104,
 /**/
     2103,