mruby-bin-mirb: fix uninitialized editor struct causing bintest failures

Zero-initialize the mirb_editor struct to prevent highlight.enabled
from containing garbage values when stdin is not a tty (e.g. in
bintest). Without this, ANSI color codes could be emitted in
non-interactive mode, breaking output string matching in tests.

Co-authored-by: Claude <noreply@anthropic.com>

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

@@ -477,6 +477,8 @@ main(int argc, char **argv)
   mirb_editor editor;
   mirb_check_data check_data;
   mrb_bool use_editor = FALSE;
+
+  memset(&editor, 0, sizeof(editor));
   mrb_ccontext *cxt;
   struct mrb_parser_state *parser;
   mrb_state *mrb;