xfree86: Do not call dlclose(NULL) [regression after ab7f057]

During unwind following an error when attempting to a load a module, we
attempt to call dlclose on a potentially NULL handle. This is a
side-effect of removing the abstraction layer in ab7f057.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Adam Jackson <ajax@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

hw/xfree86/loader/loader.c

@@ -184,7 +184,8 @@ void
 LoaderUnload(const char *name, void *handle)
 {
     xf86Msg(X_INFO, "Unloading %s\n", name);
-    dlclose(handle);
+    if (handle)
+	dlclose(handle);
 }
 
 unsigned long LoaderOptions = 0;