vim-patch:8.2.5017: gcc 12.1 warns for uninitialized variable

Problem:    Gcc 12.1 warns for uninitialized variable.
Solution:   Initialize the variable. (closes vim/vim#10476)

https://github.com/vim/vim/commit/8be36eecdc6728021f9c384c2305f114e671ec32

Co-authored-by: mityu <mityu.mail@gmail.com>

src/nvim/eval/vars.c

@@ -1369,7 +1369,7 @@ static void tv_list_unlet_range(list_T *const l, listitem_T *const li_first, con
 int do_unlet(const char *const name, const size_t name_len, const bool forceit)
   FUNC_ATTR_NONNULL_ALL
 {
-  const char *varname;
+  const char *varname = NULL;  // init to shut up gcc
   dict_T *dict;
   hashtab_T *ht = find_var_ht_dict(name, name_len, &varname, &dict);