Minor fix to attaching stencils

BUG=skia:

Review URL: https://codereview.chromium.org/1344033002
diff --git a/src/gpu/GrRenderTarget.cpp b/src/gpu/GrRenderTarget.cpp
index c16738e..4bb0143 100644
--- a/src/gpu/GrRenderTarget.cpp
+++ b/src/gpu/GrRenderTarget.cpp
@@ -69,6 +69,11 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 bool GrRenderTargetPriv::attachStencilAttachment(GrStencilAttachment* stencil) {
+    if (!stencil && !fRenderTarget->fStencilAttachment) {
+        // No need to do any work since we currently don't have a stencil attachment and
+        // we're not acctually adding one.
+        return true;
+    }
     fRenderTarget->fStencilAttachment = stencil;
     if (!fRenderTarget->completeStencilAttachment()) {
         SkSafeSetNull(fRenderTarget->fStencilAttachment);
diff --git a/src/gpu/gl/GrGLRenderTarget.cpp b/src/gpu/gl/GrGLRenderTarget.cpp
index dfb4e88..e5210fa 100644
--- a/src/gpu/gl/GrGLRenderTarget.cpp
+++ b/src/gpu/gl/GrGLRenderTarget.cpp
@@ -89,19 +89,17 @@
     const GrGLInterface* interface = gpu->glInterface();
     GrStencilAttachment* stencil = this->renderTargetPriv().getStencilAttachment();
     if (nullptr == stencil) {
-        if (this->renderTargetPriv().getStencilAttachment()) {
-            GR_GL_CALL(interface, FramebufferRenderbuffer(GR_GL_FRAMEBUFFER,
-                                                          GR_GL_STENCIL_ATTACHMENT,
-                                                          GR_GL_RENDERBUFFER, 0));
-            GR_GL_CALL(interface, FramebufferRenderbuffer(GR_GL_FRAMEBUFFER,
-                                                          GR_GL_DEPTH_ATTACHMENT,
-                                                          GR_GL_RENDERBUFFER, 0));
+        GR_GL_CALL(interface, FramebufferRenderbuffer(GR_GL_FRAMEBUFFER,
+                                                      GR_GL_STENCIL_ATTACHMENT,
+                                                      GR_GL_RENDERBUFFER, 0));
+        GR_GL_CALL(interface, FramebufferRenderbuffer(GR_GL_FRAMEBUFFER,
+                                                      GR_GL_DEPTH_ATTACHMENT,
+                                                      GR_GL_RENDERBUFFER, 0));
 #ifdef SK_DEBUG
-            GrGLenum status;
-            GR_GL_CALL_RET(interface, status, CheckFramebufferStatus(GR_GL_FRAMEBUFFER));
-            SkASSERT(GR_GL_FRAMEBUFFER_COMPLETE == status);
+        GrGLenum status;
+        GR_GL_CALL_RET(interface, status, CheckFramebufferStatus(GR_GL_FRAMEBUFFER));
+        SkASSERT(GR_GL_FRAMEBUFFER_COMPLETE == status);
 #endif
-        }
         return true;
     } else {
         const GrGLStencilAttachment* glStencil = static_cast<const GrGLStencilAttachment*>(stencil);