return null if subshader fails

src/core/SkFilterShader.cpp

Index: src/core/SkFilterShader.cpp
diff --git a/src/core/SkFilterShader.cpp b/src/core/SkFilterShader.cpp
index 5094706ea784d56e8aafe3d398f72ff4894e74bd..0c92d4c3e3f90f4677d19a32044bb48c2f026754 100644
--- a/src/core/SkFilterShader.cpp
+++ b/src/core/SkFilterShader.cpp
@@ -58,8 +58,9 @@ uint32_t SkFilterShader::FilterShaderContext::getFlags() const {
 SkShader::Context* SkFilterShader::onCreateContext(const ContextRec& rec, void* storage) const {
     char* shaderContextStorage = (char*)storage + sizeof(FilterShaderContext);
     SkShader::Context* shaderContext = fShader->createContext(rec, shaderContextStorage);
-    SkASSERT(shaderContext);
-
+    if (NULL == shaderContext) {
+        return NULL;
+    }
     return SkNEW_PLACEMENT_ARGS(storage, FilterShaderContext, (*this, shaderContext, rec));
 }