patch 8.1.1897: may free memory twice when out of memory

Problem:    May free memory twice when out of memory.
Solution:   Check that backslash_halve_save() returns a different pointer.
            (Dominique Pelle, closes #4847)
diff --git a/src/version.c b/src/version.c
index 81e0d5a..14a9528 100644
--- a/src/version.c
+++ b/src/version.c
@@ -766,6 +766,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1897,
+/**/
     1896,
 /**/
     1895,