Fix an off-by-one causing a buffer overflow in mirb.

mrbgems/mruby-bin-mirb/tools/mirb/mirb.c

@@ -442,7 +442,7 @@ main(int argc, char **argv)
     char_index = 0;
     while ((last_char = getchar()) != '\n') {
       if (last_char == EOF) break;
-      if (char_index > sizeof(last_code_line)-2) {
+      if (char_index >= sizeof(last_code_line)-2) {
         fputs("input string too long\n", stderr);
         continue;
       }