Prevent post-receive error when push to project with dead forks

If project has open merge request from fork and this fork was removed
before merge request was closed it cause exception during push

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/project.rb

@@ -411,7 +411,7 @@ class Project < ActiveRecord::Base
     mrs = self.merge_requests.opened.where(target_branch: branch_name).to_a
     mrs = mrs.select(&:last_commit).select { |mr| c_ids.include?(mr.last_commit.id) }
-    mrs.uniq.each do |merge_request|
+    mrs.uniq.select(&:source_project).each do |merge_request|
       MergeRequests::MergeService.new.execute(merge_request, user, nil)
     end
@@ -420,7 +420,7 @@ class Project < ActiveRecord::Base
     # Update code for merge requests between project and project fork
     mrs += self.fork_merge_requests.opened.by_branch(branch_name).to_a
-    mrs.uniq.each do |merge_request|
+    mrs.uniq.select(&:source_project).each do |merge_request|
       merge_request.reload_code
       merge_request.mark_as_unchecked
     end
@@ -435,7 +435,7 @@ class Project < ActiveRecord::Base
     mrs = self.origin_merge_requests.opened.where(source_branch: branch_name).to_a
     mrs += self.fork_merge_requests.opened.where(source_branch: branch_name).to_a
-    mrs.uniq.each do |merge_request|
+    mrs.uniq.select(&:source_project).each do |merge_request|
       Note.create_new_commits_note(merge_request, merge_request.project,
                                    user, commits)
     end