glx: Add null pointer protection to __glGetProcAddress

This can't happen when GLX is the backing window system, but can
elsewhere.  We may as well protect against it at a high level.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Adam Jackson <ajax@redhat.com>

glx/glxext.c

@@ -554,7 +554,9 @@ __glXsetGetProcAddress(glx_gpa_proc get_proc_address)
 
 void *__glGetProcAddress(const char *proc)
 {
-    return _get_proc_address(proc);
+    void *ret = _get_proc_address(proc);
+
+    return ret ? ret : NoopDDA;
 }
 
 /*