comp: Fix uninitialised errors from Valgrind
Conditional jump or move depends on uninitialised value(s)
at 0x6A5A7A0: vk_select_physical_device (comp_vk.c:847)
by 0x6A5AAF9: vk_create_device (comp_vk.c:916)
by 0x6A5CD82: compositor_check_vulkan_caps (comp_compositor.c:401)
by 0x6A5D659: comp_compositor_create (comp_compositor.c:656)
by 0x6A5F99B: xrt_gfx_vk_provider_create (comp_glue_vk.c:57)
by 0x6A78197: oxr_session_populate_vk (oxr_session_vk.c:32)
by 0x6A77E59: oxr_session_create_impl (oxr_session.c:536)
by 0x6A77F53: oxr_session_create (oxr_session.c:567)
by 0x6A6EA74: oxr_xrCreateSession (oxr_api_session.c:44)
by 0x2632A8: xrCreateSession (in hello_xr)
by 0x239F0D: (anonymous namespace)::OpenXrProgram::InitializeSession() (in hello_xr)
by 0x22ED25: main (in hello_xr)
Uninitialised value was created by a stack allocation
at 0x6A5CB05: compositor_check_vulkan_caps (comp_compositor.c:349)