vim-patch:8.0.1172: when E734 is given option is still set (#8988)

Problem:    When E734 is given option is still set.
Solution:   Assign NULL to "s". (Christian Brabandt)
2a6a6c3014

src/nvim/eval.c

@@ -1912,6 +1912,7 @@ static char_u *ex_let_one(char_u *arg, typval_T *const tv,
         if ((opt_type == 1 && *op == '.')
             || (opt_type == 0 && *op != '.')) {
           EMSG2(_(e_letwrong), op);
+          s = NULL;  // don't set the value
         } else {
           if (opt_type == 1) {  // number
             if (*op == '+') {