Handle invalid project ID in reply-by-email

When an invalid project ID were supplied, the reply processor would
quietly fail with an error:

```
undefined method `full_path_slug' for nil:NilClass
```

This commit checks for a nil Project and ensures the proper exception is
thrown.

Changelog: fixed

lib/gitlab/email/handler/reply_processing.rb

@@ -84,6 +84,8 @@ module Gitlab
         end
 
         def valid_project_slug?(found_project)
+          return false unless found_project
+
           project_slug == found_project.full_path_slug
         end
 

spec/lib/gitlab/email/handler/create_issue_handler_spec.rb

@@ -128,6 +128,14 @@ RSpec.describe Gitlab::Email::Handler::CreateIssueHandler do
         expect { receiver.execute }.to raise_error(Gitlab::Email::ProjectNotFound)
       end
     end
+
+    context 'when project ID is invalid' do
+      it 'raises a ProjectNotFound' do
+        handler = described_class.new(email_raw, "gitlabhq-gitlabhq-#{Gitlab::Database::MAX_INT_VALUE}-#{user.incoming_email_token}-issue")
+
+        expect { handler.execute }.to raise_error(Gitlab::Email::ProjectNotFound)
+      end
+    end
   end
 
   def email_fixture(path)