patch 8.2.3883: crash when switching to other regexp engine fails

Problem:    Crash when switching to other regexp engine fails.
Solution:   Check for regprog being NULL.
diff --git a/src/version.c b/src/version.c
index a77aa04..05b8120 100644
--- a/src/version.c
+++ b/src/version.c
@@ -750,6 +750,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3883,
+/**/
     3882,
 /**/
     3881,