Fix potential null pointer dereference.

Change-Id: I12249cc2e647296eb45f9e7f9082fe3b477b9011
Reviewed-on: https://swiftshader-review.googlesource.com/3463
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>
diff --git a/src/Shader/VertexRoutine.cpp b/src/Shader/VertexRoutine.cpp
index 61bca55..fc6aa72 100644
--- a/src/Shader/VertexRoutine.cpp
+++ b/src/Shader/VertexRoutine.cpp
@@ -52,7 +52,7 @@
 			Registers r(shader);
 			r.data = data;
 			r.constants = *Pointer<Pointer<Byte> >(data + OFFSET(DrawData,constants));
-			if(shader->instanceIdDeclared)
+			if(shader && shader->instanceIdDeclared)
 			{
 				r.instanceID = *Pointer<Int>(data + OFFSET(DrawData, instanceID));
 			}