patch 8.1.0645: Coverity warns for possible use of NULL pointer

Problem:    Coverity warns for possible use of NULL pointer.
Solution:   Check return value of vterm_obtain_screen().
diff --git a/src/version.c b/src/version.c
index cec9bc6..823db35 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    645,
+/**/
     644,
 /**/
     643,