f_spellbadword: set len=0 for non-found word

`len` is used with `list_append_string` later, and should reflect the
length of `word` (i.e. 0 when not setting word / breaking above).

Ref: neovim/neovim#9782 (comment)

src/nvim/eval.c

@@ -16764,6 +16764,7 @@ static void f_spellbadword(typval_T *argvars, typval_T *rettv, FunPtr fptr)
         }
         str += len;
         capcol -= len;
+        len = 0;
       }
     }
   }