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/charset.c

@@ -1900,7 +1900,8 @@ void backslash_halve(char_u *p)
 /// @param p
 ///
 /// @return String with the number of backslashes halved.
-char_u* backslash_halve_save(char_u *p)
+char_u *backslash_halve_save(const char_u *p)
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_NONNULL_RET
 {
   // TODO(philix): simplify and improve backslash_halve_save algorithm
   char_u *res = vim_strsave(p);

src/nvim/ex_getln.c

@@ -4212,24 +4212,24 @@ static int showmatches(expand_T *xp, int wildmenu)
             || xp->xp_context == EXPAND_BUFFERS) {
           /* highlight directories */
           if (xp->xp_numfiles != -1) {
-            char_u  *halved_slash;
+            // Expansion was done before and special characters
-            char_u  *exp_path;
+            // were escaped, need to halve backslashes.  Also
-
+            // $HOME has been replaced with ~/.
-            /* Expansion was done before and special characters
+            char_u *exp_path = expand_env_save_opt(files_found[k], true);
-             * were escaped, need to halve backslashes.  Also
+            char_u *path = exp_path != NULL ? exp_path : files_found[k];
-             * $HOME has been replaced with ~/. */
+            char_u *halved_slash = backslash_halve_save(path);
-            exp_path = expand_env_save_opt(files_found[k], TRUE);
-            halved_slash = backslash_halve_save(
-                exp_path != NULL ? exp_path : files_found[k]);
             j = os_isdir(halved_slash);
             xfree(exp_path);
-            xfree(halved_slash);
+            if (halved_slash != path) {
-          } else
+              xfree(halved_slash);
-            /* Expansion was done here, file names are literal. */
+            }
+          } else {
+            // Expansion was done here, file names are literal.
             j = os_isdir(files_found[k]);
-          if (showtail)
+          }
+          if (showtail) {
             p = L_SHOWFILE(k);
-          else {
+          } else {
             home_replace(NULL, files_found[k], NameBuff, MAXPATHL,
                 TRUE);
             p = NameBuff;

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;

src/nvim/path.c

@@ -1262,7 +1262,10 @@ int gen_expand_wildcards(int num_pat, char_u **pat, int *num_file,
       } else {
         addfile(&ga, t, flags);
       }
-      xfree(t);
+
+      if (t != p) {
+        xfree(t);
+      }
     }
 
     if (did_expand_in_path && !GA_EMPTY(&ga) && (flags & EW_PATH))