Fix warnings: eval.c: add_nr_var(): Out of bounds: FP.

Problem    : Out-of-bound array access @ 18737.
Diagnostic : False positive.
Rationale  : Situation is intentional. `dictitem_T` is a prefix all dict
             items whill share, but actual size of each item will be
             different depending on its key length. `di_key` array field
             is declared of size 1 just to have a field name, but real
             size will vary for each item.
Resolution : Make analyzer ignore it.
             This could be refactored to use C99-allowed variable length
             arrays, but eval.c is bound to dissappear, so no effort is
             done in that sense.

src/nvim/eval.c

@@ -18809,7 +18809,9 @@ free_funccal (
  */
 static void add_nr_var(dict_T *dp, dictitem_T *v, char *name, varnumber_T nr)
 {
+#ifndef __clang_analyzer__
   STRCPY(v->di_key, name);
+#endif
   v->di_flags = DI_FLAGS_RO | DI_FLAGS_FIX;
   hash_add(&dp->dv_hashtab, DI2HIKEY(v));
   v->di_tv.v_type = VAR_NUMBER;