pdfviewer: don't crash if the xref is corrupted.

experimental/PdfViewer/pdfparser/native/SkPdfNativeTokenizer.cpp

Index: experimental/PdfViewer/pdfparser/native/SkPdfNativeTokenizer.cpp
===================================================================
--- experimental/PdfViewer/pdfparser/native/SkPdfNativeTokenizer.cpp	(revision 10286)
+++ experimental/PdfViewer/pdfparser/native/SkPdfNativeTokenizer.cpp	(working copy)
@@ -291,6 +291,9 @@
 }
 
 static const unsigned char* readString(int level, const unsigned char* start, const unsigned char* end, SkPdfObject* str, SkPdfAllocator* allocator) {
+    if (!allocator) {
+        return end;
+    }
     int outLength = readStringLength(level, start, end);
     // TODO(edisonn): optimize the allocation, don't allocate new string, but put it in a preallocated buffer
     unsigned char* out = (unsigned char*)allocator->alloc(outLength);
@@ -434,6 +437,9 @@
 }
 
 static const unsigned char* readHexString(int level, const unsigned char* start, const unsigned char* end, SkPdfObject* str, SkPdfAllocator* allocator) {
+    if (!allocator) {
+        return end;
+    }
     int outLength = readHexStringLength(level, start, end);
     // TODO(edisonn): optimize the allocation, don't allocate new string, but put it in a preallocated buffer
     unsigned char* out = (unsigned char*)allocator->alloc(outLength);
@@ -556,6 +562,9 @@
 }
 
 static const unsigned char* readName(int level, const unsigned char* start, const unsigned char* end, SkPdfObject* name, SkPdfAllocator* allocator) {
+    if (!allocator) {
+        return end;
+    }
     int outLength = readNameLength(level, start, end);
     // TODO(edisonn): optimize the allocation, don't allocate new string, but put it in a preallocated buffer
     unsigned char* out = (unsigned char*)allocator->alloc(outLength);