fixing onSkipScanlines png error

src/codec/SkCodec_libpng.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 "SkCodec_libpng.h"
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
@@ skipping 606 matching lines @@
         return SkImageGenerator::kSuccess;
     }
 
     SkImageGenerator::Result onSkipScanlines(int count) override {
         // FIXME: Could we use the return value of setjmp to specify the type of
         // error?
         if (setjmp(png_jmpbuf(fCodec->fPng_ptr))) {
             SkCodecPrintf("setjmp long jump!\n");
             return SkImageGenerator::kInvalidInput;
         }
-
-        png_read_rows(fCodec->fPng_ptr, png_bytepp_NULL, png_bytepp_NULL, count);
+        //there is a potential tradeoff of memory vs speed created by putting this in a loop. 
+        //calling png_read_rows in a loop is insignificantly slower than calling it once with count 
+        //as png_read_rows has it's own loop which calls png_read_row count times.
+        for (int i = 0; i < count; i++) {
+            png_read_rows(fCodec->fPng_ptr, &fSrcRow, png_bytepp_NULL, 1);
+        }
         return SkImageGenerator::kSuccess;
     }
 
     void onFinish() override {
         fCodec->finish();
     }
 
     bool onReallyHasAlpha() const override { return fHasAlpha; }
 
 private:
@@ skipping 32 matching lines @@
 
     SkASSERT(fNumberPasses != INVALID_NUMBER_PASSES);
     if (fNumberPasses > 1) {
         // We cannot efficiently do scanline decoding.
         return NULL;
     }
 
     return SkNEW_ARGS(SkPngScanlineDecoder, (dstInfo, this));
 }