Add null-check for flymake-timer.

TBR=evanm@chromium.org
NOTRY=true

Change-Id: I14319a25baa26ed7361e796839ed6b4c689529e5
Reviewed-on: https://chromium-review.googlesource.com/1054491Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557678}

tools/emacs/flymake-chromium.el

@@ -109,7 +109,7 @@
                      'cr-flymake-getfname)))
     (flymake-find-file-hook)
     (if flymake-mode
-        (cancel-timer flymake-timer)
+        (when flymake-timer (cancel-timer flymake-timer))
       (kill-local-variable 'flymake-allowed-file-name-masks))))
 
 (defun cr-compile ()