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)

src/version.c

@@ -765,6 +765,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1897,
 /**/
     1896,
 /**/