SkPDF: do not assert when openStream return nullptr

src/pdf/SkPDFFont.cpp

Index: src/pdf/SkPDFFont.cpp
diff --git a/src/pdf/SkPDFFont.cpp b/src/pdf/SkPDFFont.cpp
index 6c37e4cde2ccc946d54ef219bce5f83790423daf..6871635da2845600f5ad43f2039cc2c7a2ce5e78 100644
--- a/src/pdf/SkPDFFont.cpp
+++ b/src/pdf/SkPDFFont.cpp
@@ -368,9 +368,11 @@ void SkPDFType0Font::getFontSubset(SkPDFCanon* canon) {
     int ttcIndex;
     std::unique_ptr<SkStreamAsset> fontAsset(face->openStream(&ttcIndex));
     size_t fontSize = fontAsset ? fontAsset->getLength() : 0;
-    SkASSERT(fontAsset);
-    SkASSERT(fontSize > 0);
-    if (fontSize > 0) {
+    if (0 == fontSize) {
+        SkDebugf("Error: (SkTypeface)(%p)::openStream() returned "
+                 "empty stream (%p) when identified as kType1CID_Font "
+                 "or kTrueType_Font.\n", face, fontAsset.get());
+    } else {
         switch (type) {
             case SkAdvancedTypefaceMetrics::kTrueType_Font: {
                 #ifdef SK_PDF_USE_SFNTLY