Have ast live longer in ISeq.compile_file to fix GC stress crash

Previously, live range of `ast_value` ended on the call right before
rb_ast_dispose(), which led to premature collection and use-after-free.

We observed this crashing on -O3, -DVM_CHECK_MODE, with GCC 11.4.0 on
Ubuntu.

Co-authored-by: Aaron Patterson <[email protected]>

Author: 2 people

iseq.c

@@ -1749,6 +1749,7 @@ iseqw_s_compile_file(int argc, VALUE *argv, VALUE self)
                                          1, NULL, 0, ISEQ_TYPE_TOP, &option,
                                          Qnil));
     rb_ast_dispose(ast);
+    RB_GC_GUARD(ast_value);
 
     rb_vm_pop_frame(ec);
     RB_GC_GUARD(v);

test/ruby/test_iseq.rb

@@ -919,4 +919,10 @@ def test_while_in_until_condition
       assert_predicate(status, :success?)
     end
   end
+
+  def test_compile_empty_under_gc_stress
+    EnvUtil.under_gc_stress do
+      RubyVM::InstructionSequence.compile_file(File::NULL)
+    end
+  end
 end