Fix a null-deref in safe_browsing::dmg::UDIFParser::ParseBlkx.

chrome/utility/safe_browsing/mac/udif.cc

 // Copyright 2015 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.
 
 #include "chrome/utility/safe_browsing/mac/udif.h"
 
 #include <CoreFoundation/CoreFoundation.h>
 #include <bzlib.h>
 #include <libkern/OSByteOrder.h>
 #include <uuid/uuid.h>
@@ skipping 476 matching lines @@
   auto* blkx = base::mac::GetValueFromDictionary<CFArrayRef>(resource_fork,
                                                              CFSTR("blkx"));
   if (!blkx) {
     DLOG(ERROR) << "No blkx entry in resource-fork";
     return false;
   }
 
   for (CFIndex i = 0; i < CFArrayGetCount(blkx); ++i) {
     auto* block_dictionary =
         base::mac::CFCast<CFDictionaryRef>(CFArrayGetValueAtIndex(blkx, i));
+    if (!block_dictionary) {
+      DLOG(ERROR) << "Skipping block " << i
+                  << " because it is not a CFDictionary";
+      continue;
+    }
+
     auto* data = base::mac::GetValueFromDictionary<CFDataRef>(block_dictionary,
                                                               CFSTR("Data"));
     if (!data) {
       DLOG(ERROR) << "Skipping block " << i
                   << " because it has no Data section";
       continue;
     }
 
     // Copy the block table out of the plist.
     std::unique_ptr<UDIFBlock> block(new UDIFBlock());
@@ skipping 350 matching lines @@
                 << chunk_->compressed_offset;
     return false;
   }
   return true;
 }
 
 }  // namespace
 
 }  // namespace dmg
 }  // namespace safe_browsing