Fix an assert statement in SkPngCodec

src/codec/SkPngCodec.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmap.h"
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
@@ skipping 543 matching lines @@
         const int height = this->getInfo().height();
         png_progressive_info_ptr callback = nullptr;
 #ifdef SK_GOOGLE3_PNG_HACK
         callback = RereadInfoCallback;
 #endif
         png_set_progressive_read_fn(this->png_ptr(), this, callback, AllRowsCallback, nullptr);
         fDst = dst;
         fRowBytes = rowBytes;
 
         fLinesDecoded = 0;
+        fFirstRow = 0;
+        fLastRow = height - 1;
 
         this->processData();
 
         if (fLinesDecoded == height) {
             return SkCodec::kSuccess;
         }
 
         if (rowsDecoded) {
             *rowsDecoded = fLinesDecoded;
         }
 
         return SkCodec::kIncompleteInput;
     }
 
     void allRowsCallback(png_bytep row, int rowNum) {
-        SkASSERT(rowNum - fFirstRow == fLinesDecoded);
+        SkASSERT(rowNum == fLinesDecoded);
         fLinesDecoded++;
         this->applyXformRow(fDst, row);
         fDst = SkTAddOffset<void>(fDst, fRowBytes);
     }
 
     void setRange(int firstRow, int lastRow, void* dst, size_t rowBytes) override {
         png_progressive_info_ptr callback = nullptr;
 #ifdef SK_GOOGLE3_PNG_HACK
         callback = RereadInfoCallback;
 #endif
@@ skipping 709 matching lines @@
     SkCodec* outCodec = nullptr;
     if (read_header(streamDeleter.get(), chunkReader, &outCodec, nullptr, nullptr)) {
         // Codec has taken ownership of the stream.
         SkASSERT(outCodec);
         streamDeleter.release();
         return outCodec;
     }
 
     return nullptr;
 }