[rubygems/rubygems] Don't crash when updating to an unsupported rubygems-update version

https://github.com/rubygems/rubygems/commit/b0badcd00a

lib/rubygems/commands/update_command.rb

@@ -288,7 +288,9 @@ command to remove old versions.
 
     installed_gems = Gem::Specification.find_all_by_name 'rubygems-update', requirement
     installed_gems = update_gem('rubygems-update', version) if installed_gems.empty?
-    version        = installed_gems.first.version
+    return if installed_gems.empty?
+
+    version = installed_gems.first.version
 
     install_rubygems version
   end

test/rubygems/test_gem_commands_update_command.rb

@@ -106,6 +106,31 @@ class TestGemCommandsUpdateCommand < Gem::TestCase
     assert_empty out
   end
 
+  def test_execute_system_when_latest_does_not_support_your_ruby
+    spec_fetcher do |fetcher|
+      fetcher.download 'rubygems-update', 9 do |s|
+        s.files = %w[setup.rb]
+        s.required_ruby_version = '> 9'
+      end
+    end
+
+    @cmd.options[:args]          = []
+    @cmd.options[:system]        = true
+
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    out = @ui.output.split "\n"
+    assert_equal "Updating rubygems-update", out.shift
+    assert_empty out
+
+    err = @ui.error.split "\n"
+    assert_equal "ERROR:  Error installing rubygems-update:", err.shift
+    assert_equal "\trubygems-update-9 requires Ruby version > 9. The current ruby version is #{Gem.ruby_version}.", err.shift
+    assert_empty err
+  end
+
   def test_execute_system_multiple
     spec_fetcher do |fetcher|
       fetcher.download 'rubygems-update', 8 do |s|