Fix SkSwizzler bug

src/codec/SkSwizzler.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 "SkCodecPriv.h"
 #include "SkColorPriv.h"
 #include "SkScaledCodec.h"
@@ skipping 675 matching lines @@
     int srcOffset = 0;
     int srcWidth = dstInfo.width();
     if (options.fSubset) {
         srcOffset = options.fSubset->left();
         srcWidth = options.fSubset->width();
     }
 
     return new SkSwizzler(proc, ctable, srcOffset, srcWidth, bpp);
 }
 
-SkSwizzler::SkSwizzler(RowProc proc, const SkPMColor* ctable, int srcOffset, int srcWidth, int bpp)
+SkSwizzler::SkSwizzler(RowProc proc, const SkPMColor* ctable, int srcOffset, int subsetWidth,
+        int bpp)
     : fRowProc(proc)
     , fColorTable(ctable)
     , fSrcOffset(srcOffset)
     , fX0(srcOffset)
-    , fSrcWidth(srcWidth)
-    , fDstWidth(srcWidth)
+    , fSubsetWidth(subsetWidth)
+    , fDstWidth(subsetWidth)
     , fSampleX(1)
     , fBPP(bpp)
 {}
 
 int SkSwizzler::onSetSampleX(int sampleX) {
     SkASSERT(sampleX > 0); // Surely there is an upper limit? Should there be
                            // way to report failure?
     fSampleX = sampleX;
     fX0 = get_start_coord(sampleX) + fSrcOffset;
-    fDstWidth = get_scaled_dimension(fSrcWidth, sampleX);
+    fDstWidth = get_scaled_dimension(fSubsetWidth, sampleX);
 
-    // check that fX0 is less than original width
-    SkASSERT(fX0 >= 0 && fX0 < fSrcWidth);
+    // check that fX0 is valid
+    SkASSERT(fX0 >= 0);
     return fDstWidth;
 }
 
 SkSwizzler::ResultAlpha SkSwizzler::swizzle(void* dst, const uint8_t* SK_RESTRICT src) {
     SkASSERT(nullptr != dst && nullptr != src);
     return fRowProc(dst, src, fDstWidth, fBPP, fSampleX * fBPP, fX0 * fBPP, fColorTable);
 }