Fix warnings: eval.c: dictitem_alloc(): Out-of-bounds access: FP.

Problem    : Out-of-bound array access @ 5737.
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

@@ -5734,7 +5734,9 @@ dict_free (
 dictitem_T *dictitem_alloc(char_u *key) FUNC_ATTR_NONNULL_RET
 {
   dictitem_T *di = xmalloc(sizeof(dictitem_T) + STRLEN(key));
+#ifndef __clang_analyzer__
   STRCPY(di->di_key, key);
+#endif
   di->di_flags = 0;
   return di;
 }