Fix integer overflow in LoadCryptInfo.

Bug: 847283
Change-Id: I7951103a5a425407b5375460a5556e8765430740
Reviewed-on: https://pdfium-review.googlesource.com/42090
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>

core/fpdfapi/parser/cpdf_security_handler.cpp

diff --git a/core/fpdfapi/parser/cpdf_security_handler.cpp b/core/fpdfapi/parser/cpdf_security_handler.cpp
index bc72ad3..f3d9201 100644
--- a/core/fpdfapi/parser/cpdf_security_handler.cpp
+++ b/core/fpdfapi/parser/cpdf_security_handler.cpp
@@ -157,6 +157,9 @@
       } else {
         nKeyBits = pEncryptDict->GetIntegerFor("Length", 256);
       }
+      if (nKeyBits < 0)
+        return false;
+
       if (nKeyBits < 40) {
         nKeyBits *= 8;
       }