Pin str content holder while enumerating lines

The content holder may be a imemo:mmtk_strbuf, and may be moved if not
pinned.

string.c

@@ -9621,6 +9621,12 @@ rb_str_enumerate_lines(int argc, VALUE *argv, VALUE str, VALUE ary)
  if (!RSTRING_LEN(str)) goto end;
  str = rb_str_new_frozen(str);
  ptr = subptr = RSTRING_PTR(str);
+#if USE_MMTK
+ volatile VALUE str_content_holder = 0;
+ if (rb_mmtk_enabled_p()) {
+ str_content_holder = rb_mmtk_string_content_holder(str);
+ }
+#endif
  pend = RSTRING_END(str);
  len = RSTRING_LEN(str);
  StringValue(rs);
@@ -9722,6 +9728,10 @@ rb_str_enumerate_lines(int argc, VALUE *argv, VALUE str, VALUE ary)
  RB_GC_GUARD(str);
  }
 
+#if USE_MMTK
+ RB_GC_GUARD(str_content_holder);
+#endif
+
  end:
  if (ary)
  return ary;