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

experimental/PdfViewer/SkPdfFont.cpp

 #include "SkPdfFont.h"
 
 #include "SkStream.h"
 #include "SkTypeface.h"
 #include "SkPdfNativeTokenizer.h"
 
 std::map<std::string, SkPdfStandardFontEntry>& getStandardFonts() {
     static std::map<std::string, SkPdfStandardFontEntry> gPdfStandardFonts;
 
     // TODO (edisonn): , vs - ? what does it mean?
@@ skipping 179 matching lines @@
 
 SkPdfFont* fontFromName(SkNativeParsedPDF* doc, SkPdfObject* obj, const char* fontName) {
     SkTypeface* typeface = SkTypefaceFromPdfStandardFont(fontName, false, false);
     if (typeface != NULL) {
         return new SkPdfStandardFont(typeface);
     }
 
     // TODO(edisonn): perf - make a map
     for (unsigned int i = 0 ; i < doc->objects(); i++) {
         SkPdfObject* obj = doc->object(i);
-        if (!obj->isDictionary()) {
+        if (!obj || !obj->isDictionary()) {
             continue;
         }
 
         SkPdfFontDescriptorDictionary* fd = obj->asDictionary()->asFontDescriptorDictionary();
 
         if (!fd->valid()) {
             continue;
         }
 
         if (fd->has_FontName() && fd->FontName(doc) == fontName) {
@@ skipping 233 matching lines @@
 SkPdfEncoding* SkPdfEncoding::fromName(const char* name) {
     SkPdfEncoding* encoding = getStandardEncodings()[name];
 
 #ifdef PDF_TRACE
     if (encoding == NULL) {
         printf("Encoding not found: %s\n", name);
     }
 #endif
     return encoding;
 }