Fix texture allocation in SWMaskHelper

src/gpu/GrGpu.cpp

Index: src/gpu/GrGpu.cpp
===================================================================
--- src/gpu/GrGpu.cpp	(revision 10193)
+++ src/gpu/GrGpu.cpp	(working copy)
@@ -211,6 +211,7 @@
         art.set(this->drawState(), renderTarget);
     }
     if (NULL == this->getDrawState().getRenderTarget()) {
+        GrAssert(0);
         return;
     }
     this->handleDirtyContext();