change mrb_bug to get mrb_state and %S formatter

src/error.c

@@ -327,15 +327,17 @@ mrb_warn(mrb_state *mrb, const char *fmt, ...)
 }
 
 void
-mrb_bug(const char *fmt, ...)
+mrb_bug(mrb_state *mrb, const char *fmt, ...)
 {
 #ifdef ENABLE_STDIO
-  va_list args;
+  va_list ap;
+  mrb_value str;
 
-  va_start(args, fmt);
-  printf("bug: ");
-  vprintf(fmt, args);
-  va_end(args);
+  va_start(ap, fmt);
+  str = mrb_vformat(mrb, fmt, ap);
+  fputs("bug: ", stderr);
+  fwrite(RSTRING_PTR(str), RSTRING_LEN(str), 1, stderr);
+  va_end(ap);
 #endif
   exit(EXIT_FAILURE);
 }

src/parse.y

@@ -3209,7 +3209,7 @@ backref_error(parser_state *p, node *n)
   } else if (c == NODE_BACK_REF) {
     yyerror_i(p, "can't set variable $%c", (int)(intptr_t)n->cdr);
   } else {
-    mrb_bug("Internal error in backref_error() : n=>car == %d", c);
+    mrb_bug(p->mrb, "Internal error in backref_error() : n=>car == %d", c);
   }
 }