Add Coccinelle patch for replacing NULL/non-NULL tt_assert().

This patch replaces tt_assert() checks for NULL/non-NULL values with
tt_ptr_op().

scripts/coccinelle/test_assert_null.cocci

@@ -0,0 +1,11 @@
+@@
+expression * e;
+@@
+
+(
+- tt_assert(e != NULL)
++ tt_ptr_op(e, OP_NE, NULL)
+|
+- tt_assert(e == NULL)
++ tt_ptr_op(e, OP_EQ, NULL)
+)