[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

1 files changed, 9 insertions, 9 deletions

diff --git a/lib/rubygems/resolver/installer_set.rb b/lib/rubygems/resolver/installer_set.rb
index 237bc3f782..45252ed241 100644
--- a/lib/rubygems/resolver/installer_set.rb
+++ b/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
+        ensure_required_ruby_version_met(newest.spec)
+        ensure_required_rubygems_version_met(newest.spec)
       else
         newest = found_matching_metadata
       end