Improve object type checking for /CIDToGIDMap.

In CPDF_CIDFont::Load(), make sure the object is either a stream or a
name. i.e. Disallow strings.

Change-Id: Id69d7953b79fdf0774e7f8ad36a5ef632c2f262b
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/71913
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fpdfapi/font/cpdf_cidfont.cpp

diff --git a/core/fpdfapi/font/cpdf_cidfont.cpp b/core/fpdfapi/font/cpdf_cidfont.cpp
index 59326c7..41b267f 100644
--- a/core/fpdfapi/font/cpdf_cidfont.cpp
+++ b/core/fpdfapi/font/cpdf_cidfont.cpp
@@ -403,7 +403,8 @@
     if (pMapStream) {
       m_pStreamAcc = pdfium::MakeRetain<CPDF_StreamAcc>(pMapStream);
       m_pStreamAcc->LoadAllDataFiltered();
-    } else if (m_pFontFile && pmap->GetString() == "Identity") {
+    } else if (m_pFontFile && pmap->IsName() &&
+               pmap->GetString() == "Identity") {
       m_bCIDIsGID = true;
     }
   }