Fix memory leak in parser when loading non-ASCII file

When loading a non-ASCII compatible file, an error is raised which
causes memory leak.

For example:

    require "tempfile"

    Tempfile.create do |f|
      f.write("# -*- coding: UTF-16BE -*-")
      f.flush

      10.times do
        20_000.times do
          begin
            load(f.path)
          rescue
          end
        end

        puts `ps -o rss= -p #{$$}`
      end
    end

Before:

    33904
    49072
    64528
    79216
    94576
    109504
    124768
    139536
    154928
    170256

After:

    19568
    21296
    21664
    21728
    22192
    22256
    22416
    22272
    22272
    22272

Author: peterzhu2118

parse.y

@@ -9417,6 +9417,10 @@ parser_set_encode(struct parser_params *p, const char *name)
         rb_ary_unshift(excargs[2], rb_sprintf("%"PRIsVALUE":%d", p->ruby_sourcefile_string, p->ruby_sourceline));
         VALUE exc = rb_make_exception(3, excargs);
         ruby_show_error_line(p, exc, &(YYLTYPE)RUBY_INIT_YYLLOC(), p->ruby_sourceline, p->lex.lastline);
+
+        rb_ast_free(p->ast);
+        p->ast = NULL;
+
         rb_exc_raise(exc);
     }
     enc = rb_enc_from_index(idx);