Remove some checks in IsLinearizedHeaderValid().

One check can never fail. The other check can be done earlier, before
creating the CPDF_LinearizedHeader.

Change-Id: I0bccb2a9e19e0d5517daf96684adba6bb3a203bf
Reviewed-on: https://pdfium-review.googlesource.com/39412
Reviewed-by: Art Snake <art-snake@yandex-team.ru>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fpdfapi/parser/cpdf_linearized_header.cpp

diff --git a/core/fpdfapi/parser/cpdf_linearized_header.cpp b/core/fpdfapi/parser/cpdf_linearized_header.cpp
index 8286e0f..da7a7ad 100644
--- a/core/fpdfapi/parser/cpdf_linearized_header.cpp
+++ b/core/fpdfapi/parser/cpdf_linearized_header.cpp
@@ -41,11 +41,9 @@
                              FX_FILESIZE document_size) {
   ASSERT(header);
   return header->GetFileSize() == document_size &&
-         static_cast<int>(header->GetFirstPageNo()) >= 0 &&
          header->GetFirstPageNo() < kMaxInt &&
          header->GetFirstPageNo() < header->GetPageCount() &&
          header->GetMainXRefTableFirstEntryOffset() < document_size &&
-         header->GetPageCount() > 0 &&
          header->GetFirstPageEndOffset() < document_size &&
          header->GetLastXRefOffset() < document_size &&
          header->GetHintStart() < document_size;
@@ -65,7 +63,7 @@
       !IsValidNumericDictionaryValue<FX_FILESIZE>(pDict.get(), "L", 1) ||
       !IsValidNumericDictionaryValue<uint32_t>(pDict.get(), "P", 0, false) ||
       !IsValidNumericDictionaryValue<FX_FILESIZE>(pDict.get(), "T", 1) ||
-      !IsValidNumericDictionaryValue<uint32_t>(pDict.get(), "N", 0) ||
+      !IsValidNumericDictionaryValue<uint32_t>(pDict.get(), "N", 1) ||
       !IsValidNumericDictionaryValue<FX_FILESIZE>(pDict.get(), "E", 1) ||
       !IsValidNumericDictionaryValue<uint32_t>(pDict.get(), "O", 1)) {
     return nullptr;