Fixed access violation error in TSFTextStore::GetScreenExt.

In some interactive_ui_tests, upon exit the TextInputClient object becomes null.
This happens mostly in release builds where the GetScreenExt call from TSF happens
after the test has exited which results in an AV in TSFTextStore::GetScreenExt.

Bug: 1053769
Change-Id: I759eac57ddf0c08988dbb88f02dc796ede92a060
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2063673Reviewed-by: Yohei Yukawa <yukawa@chromium.org>
Commit-Queue: Anupam Snigdha <snianu@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#742416}

ui/base/ime/win/tsf_text_store.cc

@@ -182,6 +182,8 @@ STDMETHODIMP TSFTextStore::GetScreenExt(TsViewCookie view_cookie, RECT* rect) {
     return E_INVALIDARG;
   if (!rect)
     return E_INVALIDARG;
+  if (!text_input_client_)
+    return E_UNEXPECTED;
 
   // {0, 0, 0, 0} means that the document rect is not currently displayed.
   SetRect(rect, 0, 0, 0, 0);