Fix crash on load *mrb

If call mrb_dump_irep_binary() with start irep with non-zero , load/run crashes.

test code:
```
#include <stdio.h>
#include <string.h>

#include "mruby.h"
#include "mruby/dump.h"
#include "mruby/compile.h"
#include "mruby/proc.h"

void dumpMRB(mrb_state *mrb, const char *filename, const char *code){
    mrbc_context *cxt;
    
    cxt = mrbc_context_new(mrb);
    cxt->capture_errors = 1;
    //cxt->no_exec = 1;
    cxt->filename = (char *)filename;
    
    struct mrb_parser_state *parser = mrb_parser_new(mrb);
    parser->s = code;
    parser->send = code + strlen(code);
    //parser->lineno = 1;
    mrb_parser_parse(parser, cxt);
    
    int n = mrb_generate_code(mrb,parser);
    if (n < 0) {
        printf("failed to generate code. err=%d\n",n);
        return;
    }
    
    FILE *f = fopen(filename,"w+b");
    int ret = mrb_dump_irep_binary(mrb, n, 0, f);
    if (ret != MRB_DUMP_OK){
        printf("failed to create %s\n",filename);
        fclose(f);
        return;
    }
    fclose(f);
    mrb_parser_free(parser);
    mrbc_context_free(mrb,cxt);
    
}

void runMRB(mrb_state *mrb, const char *filename){
    FILE *f = fopen(filename, "rb");
    int n = mrb_read_irep_file(mrb, f);
    
    if (n < 0){
        printf("failed to load mrb:%s\n", filename);
        return;
    }
    mrb_run(mrb, mrb_proc_new(mrb, mrb->irep[n]), mrb_top_self(mrb));
    fclose(f);
}

int main(int argc, const char * argv[])
{
    mrb_state *mrb1,*mrb2;
    mrb1 = mrb_open();
    mrb2 = mrb_open();
    dumpMRB(mrb1,"one.mrb","puts :one; def func1;puts :func1;end");
    dumpMRB(mrb1,"two.mrb","puts :two; func1 ");
    runMRB(mrb2, "one.mrb");
    runMRB(mrb2, "two.mrb");
    
    mrb_close(mrb1);
    mrb_close(mrb2);
    return 0;
}


```