Fix segfault for update --count=1 (#10)

from ejoffe#423

pr:updateC1Segfault

---

**Stack**:
-   #13
-   #12
- ➡ #10


⚠️ *Part of a stack created by [spr](https://github.com/ejoffe/spr). Do
not merge manually using the UI - doing so may have unexpected results.*

spr/spr.go

@@ -468,7 +468,7 @@ func sortPullRequestsByLocalCommitOrder(pullRequests []*github.PullRequest, loca
 
 	var sortedPullRequests []*github.PullRequest
 	for _, commit := range localCommits {
-		if !commit.WIP {
+		if !commit.WIP && pullRequestMap[commit.CommitID] != nil {
 			sortedPullRequests = append(sortedPullRequests, pullRequestMap[commit.CommitID])
 		}
 	}