Update copy of safe_math_impl.h to take a fix from upstream:

third_party/base/numerics/safe_math_impl.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef PDFIUM_THIRD_PARTY_SAFE_MATH_IMPL_H_
 #define PDFIUM_THIRD_PARTY_SAFE_MATH_IMPL_H_
 
 #include <stdint.h>
 
 #include <cmath>
@@ skipping 160 matching lines @@
       static_cast<IntermediateType>(x) * static_cast<IntermediateType>(y);
   *validity = DstRangeRelationToSrcRange<T>(tmp);
   return static_cast<T>(tmp);
 }
 
 template <typename T>
 typename enable_if<std::numeric_limits<T>::is_integer&& std::numeric_limits<
                        T>::is_signed&&(sizeof(T) * 2 > sizeof(uintmax_t)),
                    T>::type
 CheckedMul(T x, T y, RangeConstraint* validity) {
-  // if either side is zero then the result will be zero.
-  if (!(x || y)) {
+  // If either side is zero then the result will be zero.
+  if (!x || !y) {
     return RANGE_VALID;
 
   } else if (x > 0) {
     if (y > 0)
       *validity =
           x <= std::numeric_limits<T>::max() / y ? RANGE_VALID : RANGE_OVERFLOW;
     else
       *validity = y >= std::numeric_limits<T>::min() / x ? RANGE_VALID
                                                          : RANGE_UNDERFLOW;
 
@@ skipping 302 matching lines @@
                             StaticDstRangeRelationToSrcRange<T, Rhs>::value !=
                                 NUMERIC_RANGE_CONTAINED &&
                             sizeof(T) >= (2 * sizeof(Rhs));
 };
 
 }  // namespace internal
 }  // namespace base
 }  // namespace pdfium
 
 #endif  // PDFIUM_THIRD_PARTY_SAFE_MATH_IMPL_H_