patch 8.1.1845: may use NULL pointer when running out of memory

Problem:    May use NULL pointer when running out of memory.
Solution:   Do not clear popup buffers when NULL. (closes #4802)
diff --git a/src/version.c b/src/version.c
index f998c52..71f6c34 100644
--- a/src/version.c
+++ b/src/version.c
@@ -770,6 +770,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1845,
+/**/
     1844,
 /**/
     1843,