spirv: Initialize spirv_test member shader.

Fix defect reported by Coverity Scan.

Uninitialized pointer field (UNINIT_CTOR)
uninit_member: Non-static class member shader 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/6640>

src/compiler/spirv/tests/helpers.h

@@ -30,6 +30,7 @@
 class spirv_test : public ::testing::Test {
 protected:
    spirv_test()
+   : shader(NULL)
    {
       glsl_type_singleton_init_or_ref();
    }