Bug 774728 - Do a null pointer check before using any gradient shaders in DrawTargetSkia r=mattwoodrow

--HG--
extra : rebase_source : 7db2d28692a3cd9cccbd542ca752950f5df90511

gfx/2d/DrawTargetSkia.cpp

@@ -206,10 +206,14 @@ void SetPaintPattern(SkPaint& aPaint, const Pattern& aPattern, Float aAlpha = 1.
                                                           &stops->mPositions.front(), 
                                                           stops->mCount, 
                                                           mode);
-        SkMatrix mat;
-        GfxMatrixToSkiaMatrix(pat.mMatrix, mat);
-        shader->setLocalMatrix(mat);
-        SkSafeUnref(aPaint.setShader(shader));
+
+        if (shader) {
+            SkMatrix mat;
+            GfxMatrixToSkiaMatrix(pat.mMatrix, mat);
+            shader->setLocalMatrix(mat);
+            SkSafeUnref(aPaint.setShader(shader));
+        }
+
       } else {
         aPaint.setColor(SkColorSetARGB(0, 0, 0, 0));
       }
@@ -233,10 +237,13 @@ void SetPaintPattern(SkPaint& aPaint, const Pattern& aPattern, Float aAlpha = 1.
                                                                   &stops->mPositions.front(), 
                                                                   stops->mCount, 
                                                                   mode);
-        SkMatrix mat;
-        GfxMatrixToSkiaMatrix(pat.mMatrix, mat);
-        shader->setLocalMatrix(mat);
-        SkSafeUnref(aPaint.setShader(shader));
+        if (shader) {
+            SkMatrix mat;
+            GfxMatrixToSkiaMatrix(pat.mMatrix, mat);
+            shader->setLocalMatrix(mat);
+            SkSafeUnref(aPaint.setShader(shader));
+        }
+
       } else {
         aPaint.setColor(SkColorSetARGB(0, 0, 0, 0));
       }