Fix an assert statement in SkPngCodec

src/codec/SkPngCodec.cpp

Index: src/codec/SkPngCodec.cpp
diff --git a/src/codec/SkPngCodec.cpp b/src/codec/SkPngCodec.cpp
index 1e633eaaed3b76b5e9aae380e8a1605b498d84cf..0675653b82da9b86cd279f088ccc15e0a581743e 100644
--- a/src/codec/SkPngCodec.cpp
+++ b/src/codec/SkPngCodec.cpp
@@ -561,6 +561,8 @@ private:
         fRowBytes = rowBytes;
 
         fLinesDecoded = 0;
+        fFirstRow = 0;
+        fLastRow = height - 1;
 
         this->processData();
 
@@ -576,7 +578,7 @@ private:
     }
 
     void allRowsCallback(png_bytep row, int rowNum) {
-        SkASSERT(rowNum - fFirstRow == fLinesDecoded);
+        SkASSERT(rowNum == fLinesDecoded);
         fLinesDecoded++;
         this->applyXformRow(fDst, row);
         fDst = SkTAddOffset<void>(fDst, fRowBytes);