nir/tests: Initialize nir_serialize_test member dup.

Fix defect reported by Coverity Scan.

Uninitialized pointer field (UNINIT_CTOR)
uninit_member: Non-static class member dup is not initialized in
this constructor nor in any functions that it calls.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8168>

src/compiler/nir/tests/serialize_tests.cpp

@@ -44,7 +44,7 @@ protected:
 };
 
 nir_serialize_test::nir_serialize_test()
-:  options()
+:  dup(NULL), options()
 {
    glsl_type_singleton_init_or_ref();