detect if the scaledimagecache returns a purged bitmap

src/core/SkBitmapScaler.cpp

 #include "SkBitmapScaler.h"
 #include "SkBitmapFilter.h"
 #include "SkRect.h"
 #include "SkTArray.h"
 #include "SkErrorInternals.h"
 #include "SkConvolver.h"
 
 // SkResizeFilter ----------------------------------------------------------------
 
 // Encapsulates computation and storage of the filters required for one complete
@@ skipping 283 matching lines @@
         return false;
     }
 
     BGRAConvolve2D(sourceSubset, static_cast<int>(source.rowBytes()),
         !source.isOpaque(), filter.xFilter(), filter.yFilter(),
         static_cast<int>(result.rowBytes()),
         static_cast<unsigned char*>(result.getPixels()),
         convolveProcs, true);
 
     *resultPtr = result;
+    resultPtr->lockPixels();
+    SkASSERT(NULL != resultPtr->getPixels());
     return true;
 }
 
 // static
 bool SkBitmapScaler::Resize(SkBitmap* resultPtr,
                             const SkBitmap& source,
                             ResizeMethod method,
                             int destWidth, int destHeight,
                             const SkConvolutionProcs& convolveProcs,
                             SkBitmap::Allocator* allocator) {
     SkIRect destSubset = { 0, 0, destWidth, destHeight };
     return Resize(resultPtr, source, method, destWidth, destHeight, destSubset,
                   convolveProcs, allocator);
 }