Fix a crash in SigninManager.finishSignIn()

No test because SigninManager is low on my list of things to write tests
for. I've filed http://crbug.com/481099 to track it.

BUG=478597

Review URL: https://codereview.chromium.org/1100983005

Cr-Commit-Position: refs/heads/master@{#327182}

chrome/android/java/src/org/chromium/chrome/browser/signin/SigninManager.java

@@ -395,6 +395,11 @@ public class SigninManager {
     }
 
     private void finishSignIn(AccountIdsAndNames accountIdsAndNames) {
+        if (mSignInAccount == null) {
+            Log.w(TAG, "Sign in request was canceled; aborting finishSignIn().");
+            return;
+        }
+
         // Cache the signed-in account name.
         ChromeSigninController.get(mContext).setSignedInAccountName(mSignInAccount.name);