allow NULL (no variable) in lvar section of mrb format; fix #2294

This fix use UINT16_MAX for NULL symbol tag, that means maximum symbol length
is not UINT16_MAX-1.

include/mruby/dump.h

@@ -94,6 +94,8 @@ struct rite_section_lv_header {
   RITE_SECTION_HEADER;
 };
 
+#define RITE_LV_NULL_MARK              UINT16_MAX
+
 struct rite_binary_footer {
   RITE_SECTION_HEADER;
 };

src/dump.c

@@ -703,6 +703,7 @@ create_lv_sym_table(mrb_state *mrb, const mrb_irep *irep, mrb_sym **syms, uint32
 
   for (i = 0; i + 1 < irep->nlocals; ++i) {
     mrb_sym const name = irep->lv[i].name;
+    if (name == 0) continue;
     if (find_filename_index(*syms, *syms_len, name) != -1) continue;
 
     ++(*syms_len);
@@ -744,12 +745,18 @@ write_lv_record(mrb_state *mrb, const mrb_irep *irep, uint8_t **start, mrb_sym c
   size_t i;
 
   for (i = 0; i + 1 < irep->nlocals; ++i) {
+    if (irep->lv[i].name == 0) {
+      cur += uint16_to_bin(RITE_LV_NULL_MARK, cur);
+      cur += uint16_to_bin(0, cur);
+    }
+    else {
       int const sym_idx = find_filename_index(syms, syms_len, irep->lv[i].name);
       mrb_assert(sym_idx != -1); /* local variable name must be in syms */
 
       cur += uint16_to_bin(sym_idx, cur);
       cur += uint16_to_bin(irep->lv[i].r, cur);
     }
+  }
 
   for (i = 0; i < irep->rlen; ++i) {
     write_lv_record(mrb, irep->reps[i], &cur, syms, syms_len);

src/load.c

@@ -411,12 +411,18 @@ read_lv_record(mrb_state *mrb, const uint8_t *start, mrb_irep *irep, size_t *rec
   for (i = 0; i + 1< irep->nlocals; ++i) {
     uint16_t const sym_idx = bin_to_uint16(bin);
     bin += sizeof(uint16_t);
+    if (sym_idx == RITE_LV_NULL_MARK) {
+      irep->lv[i].name = 0;
+      irep->lv[i].r = 0;
+    }
+    else {
       if (sym_idx >= syms_len) {
         return MRB_DUMP_GENERAL_FAILURE;
       }
       irep->lv[i].name = syms[sym_idx];
 
       irep->lv[i].r = bin_to_uint16(bin);
+    }
     bin += sizeof(uint16_t);
   }