Prevent segfault in String#scan with ObjectSpace.each_object

Calling `String#scan` without a block creates an incomplete MatchData
object whose `RMATCH(match)->str` is Qfalse. Usually this object is not
leaked, but it was possible to pull it by using ObjectSpace.each_object.

This change hides the internal MatchData object by using rb_obj_hide.

Fixes [Bug #19159]

re.c

@@ -1739,6 +1739,13 @@ rb_reg_search_set_match(VALUE re, VALUE str, long pos, int reverse, int set_back
     if (set_backref_str) {
         RMATCH(match)->str = rb_str_new4(str);
     }
+    else {
+        /* Note that a MatchData object with RMATCH(match)->str == 0 is incomplete!
+         * We need to hide the object from ObjectSpace.each_object.
+         * https://bugs.ruby-lang.org/issues/19159
+         */
+        rb_obj_hide(match);
+    }
 
     RMATCH(match)->regexp = re;
     rb_backref_set(match);

test/ruby/test_string.rb

@@ -1601,6 +1601,15 @@ CODE
     assert_equal(%w[1 2 3], S("a1 a2 a3").scan(/a\K./))
   end
 
+  def test_scan_segv
+    bug19159 = '[Bug #19159]'
+    assert_nothing_raised(Exception, bug19159) do
+      ObjectSpace.each_object(MatchData).to_a
+      "".scan(//)
+      ObjectSpace.each_object(MatchData).to_a.inspect
+    end
+  end
+
   def test_size
     assert_equal(0, S("").size)
     assert_equal(4, S("1234").size)