[rubygems/rubygems] Fix gem install <non-existent-gem> --force crash

Before:

```
$ gem install sfdsfdsfsdide --force
ERROR:  While executing gem ... (NoMethodError)
    undefined method `spec' for nil:NilClass

    @always_install << newest.spec
                             ^^^^^
```

After:

```
$ gem install sfdsfdsfsdide --force
ERROR:  Could not find a valid gem 'sfdsfdsfsdide' (>= 0) in any repository
```

https://github.com/rubygems/rubygems/commit/4e2bfd1101

lib/rubygems/resolver/installer_set.rb

@@ -76,21 +76,21 @@ class Gem::Resolver::InstallerSet < Gem::Resolver::Set
 
     newest = found.last
 
+    unless newest
+      exc = Gem::UnsatisfiableDependencyError.new request
+      exc.errors = errors
+
+      raise exc
+    end
+
     unless @force
       found_matching_metadata = found.reverse.find do |spec|
         metadata_satisfied?(spec)
       end
 
       if found_matching_metadata.nil?
-        if newest
         ensure_required_ruby_version_met(newest.spec)
         ensure_required_rubygems_version_met(newest.spec)
-        else
-          exc = Gem::UnsatisfiableDependencyError.new request
-          exc.errors = errors
-
-          raise exc
-        end
       else
         newest = found_matching_metadata
       end

test/rubygems/test_gem_commands_install_command.rb

@@ -277,6 +277,22 @@ ERROR:  Could not find a valid gem 'bar' (= 0.5) (required by 'foo' (>= 0)) in a
     assert_match(/ould not find a valid gem 'nonexistent'/, @ui.error)
   end
 
+  def test_execute_nonexistent_force
+    spec_fetcher
+
+    @cmd.options[:args] = %w[nonexistent]
+    @cmd.options[:force] = true
+
+    use_ui @ui do
+      e = assert_raise Gem::MockGemUi::TermError do
+        @cmd.execute
+      end
+      assert_equal 2, e.exit_code
+    end
+
+    assert_match(/ould not find a valid gem 'nonexistent'/, @ui.error)
+  end
+
   def test_execute_dependency_nonexistent
     spec_fetcher do |fetcher|
       fetcher.spec 'foo', 2, 'bar' => '0.5'