pdfviewer: more simple fixed to prevent crashes

experimental/PdfViewer/pdfparser/native/SkNativeParsedPDF.cpp

 #include "SkNativeParsedPDF.h"
 #include "SkPdfNativeTokenizer.h"
 #include "SkPdfBasics.h"
 #include "SkPdfObject.h"
 
 #include <stdio.h>
 #include <string.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 
@@ skipping 171 matching lines @@
     }
     // TODO(edisonn): it should never get here? there is no trailer?
     return current;
 }
 
 long SkNativeParsedPDF::readTrailer(const unsigned char* trailerStart, const unsigned char* trailerEnd, bool storeCatalog) {
     SkPdfObject trailerKeyword;
     // TODO(edisonn): use null allocator, and let it just fail if memory
     // needs allocated (but no crash)!
     const unsigned char* current =
-            nextObject(0, trailerStart, trailerEnd, &trailerKeyword, fAllocator, NULL);
+            nextObject(0, trailerStart, trailerEnd, &trailerKeyword, NULL, NULL);
 
-    if (strlen("trailer") != trailerKeyword.len() &&
+    if (!trailerKeyword.isKeyword() || strlen("trailer") != trailerKeyword.len() ||
         strncmp(trailerKeyword.c_str(), "trailer", strlen("trailer")) != 0) {
         // TODO(edisonn): report warning, rebuild trailer from objects.
         return -1;
     }
 
     SkPdfObject token;
     current = nextObject(0, current, trailerEnd, &token, fAllocator, NULL);
     if (!token.isDictionary()) {
         return -1;
     }
@@ skipping 231 matching lines @@
     return (SkPdfObject*)ref;
 }
 
 size_t SkNativeParsedPDF::bytesUsed() const {
     return fAllocator->bytesUsed() +
            fContentLength +
            fObjects.count() * sizeof(PublicObjectEntry) +
            fPages.count() * sizeof(SkPdfPageObjectDictionary*) +
            sizeof(*this);
 }