vim-patch:8.1.1897: may free memory twice when out of memory (#10827)

Problem:    May free memory twice when out of memory.
Solution:   Check that backslash_halve_save() returns a different pointer.
            (Dominique Pelle, closes vim/vim#4847)
f1552d07d7

src/nvim/os/env.c

@@ -412,6 +412,7 @@ void expand_env_esc(char_u *restrict srcp,
                     bool esc,
                     bool one,
                     char_u *prefix)
+  FUNC_ATTR_NONNULL_ARG(1, 2)
 {
   char_u      *tail;
   char_u      *var;