PullRequest: Fix an exception on closed request.
PullRequest: Fix an exception on closed request.

While showing a closed pull request, an exception could be thrown if the
branch requested from is deleted.

violetag/demo,ahb0327/yobi,oolso/yobi,naver/yobi,yona-projects/yona,doortts/fork-yobi,yona-projects/yona,ihoneymon/yobi,bloodybear/yona,violetag/demo,bloodybear/yona,ChangsungKim/TestRepository01,ihoneymon/yobi,bloodybear/yona,doortts/fork-yobi,ahb0327/yobi,ChangsungKim/TestRepository01,doortts/fork-yobi,brainagenet/yobi,doortts/forked-for-history,Limseunghwan/oss,doortts/forked-for-history,ihoneymon/yobi,yona-projects/yona,brainagenet/yobi,yona-projects/yona,ahb0327/yobi,doortts/fork-yobi,naver/yobi,bloodybear/yona,brainagenet/yobi,doortts/forked-for-history,oolso/yobi,Limseunghwan/oss,Limseunghwan/oss,naver/yobi,oolso/yobi