Avoid requiring CIDSET_UNKNOWN be 0 in CPDF_ToUnicodeMap::Load()

It happens to be that way, but it is better if we don't rely on it.

Change-Id: I732b86deb7f3176fb6c1567db9609e0721da1e8e
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/88071
Auto-Submit: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fpdfapi/font/cpdf_tounicodemap.cpp

diff --git a/core/fpdfapi/font/cpdf_tounicodemap.cpp b/core/fpdfapi/font/cpdf_tounicodemap.cpp
index 348cdb8..d1df1f8 100644
--- a/core/fpdfapi/font/cpdf_tounicodemap.cpp
+++ b/core/fpdfapi/font/cpdf_tounicodemap.cpp
@@ -136,7 +136,7 @@
     else if (word == "/Adobe-GB1-UCS2")
       cid_set = CIDSET_GB1;
   }
-  if (cid_set) {
+  if (cid_set != CIDSET_UNKNOWN) {
     auto* manager = CPDF_FontGlobals::GetInstance()->GetCMapManager();
     m_pBaseMap = manager->GetCID2UnicodeMap(cid_set);
   }