Avoid an undefined shift in ReadPageHintTable().

core/fpdfapi/fpdf_parser/cpdf_hint_tables.cpp

Index: core/fpdfapi/fpdf_parser/cpdf_hint_tables.cpp
diff --git a/core/fpdfapi/fpdf_parser/cpdf_hint_tables.cpp b/core/fpdfapi/fpdf_parser/cpdf_hint_tables.cpp
index 63a64a302691f5b8ca63cdf1b1f674cd437d8c4a..7a3fbacfb3ae91b71b9ca87da99b2edcb44ecb69 100644
--- a/core/fpdfapi/fpdf_parser/cpdf_hint_tables.cpp
+++ b/core/fpdfapi/fpdf_parser/cpdf_hint_tables.cpp
@@ -109,6 +109,12 @@ FX_BOOL CPDF_HintTables::ReadPageHintTable(CFX_BitStream* hStream) {
   // Item 13: Skip Item 13 which has 16 bits.
   hStream->SkipBits(16);
 
+  // The maximum number of bits allowed to represent the greatest number of
+  // shared object references. 2^39 should be more than enough.
+  constexpr uint32_t kMaxSharedObjBits = 39;
+  if (dwSharedObjBits > kMaxSharedObjBits)
+    return FALSE;
+
   CPDF_Object* pPageNum = m_pLinearizedDict->GetDirectObjectBy("N");
   int nPages = pPageNum ? pPageNum->GetInteger() : 0;
   if (nPages < 1)