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 672 matching lines @@
         return nullptr;
     }
 
     // Store deltaSrc in bytes if it is an even multiple, otherwise use bits
     int deltaSrc = SkIsAlign8(BitsPerPixel(sc)) ? BytesPerPixel(sc) : BitsPerPixel(sc);
 
     return new SkSwizzler(proc, ctable, deltaSrc, dstInfo.width());
 }
 
 SkSwizzler::SkSwizzler(RowProc proc, const SkPMColor* ctable,
-                       int deltaSrc, int srcWidth)
+                       int deltaSrc, int subsetWidth)
     : fRowProc(proc)
     , fColorTable(ctable)
     , fDeltaSrc(deltaSrc)
-    , fSrcWidth(srcWidth)
-    , fDstWidth(srcWidth)
+    , fSubsetWidth(subsetWidth)
+    , fDstWidth(subsetWidth)
     , fSampleX(1)
     , fX0(0)
 {}
 
 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);
-    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, fDeltaSrc, fSampleX * fDeltaSrc,
             fX0 * fDeltaSrc, fColorTable);
 }