glx_arb_create_context: Initialize variable.

Fix defect reported by Coverity Scan.

Uninitialized pointer read (UNINIT) uninit_use_in_call: Using uninitialized value ctx when calling glXDestroyContext.

Fixes: 4991384f ("glx: add test for GLX_ARB_create_context_no_error") Signed-off-by: Vinson Lee vlee@freedesktop.org