st/nine: Fix possible crash on error

In case swapchain creation fails This->swapchains[i] might be NULL and
causes a crash.

Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Axel Davy <axel.davy@ens.fr>

src/gallium/state_trackers/nine/device9.c

@@ -460,7 +460,8 @@ NineDevice9_dtor( struct NineDevice9 *This )
 
     if (This->swapchains) {
         for (i = 0; i < This->nswapchains; ++i)
-            NineUnknown_Unbind(NineUnknown(This->swapchains[i]));
+            if (This->swapchains[i])
+                NineUnknown_Unbind(NineUnknown(This->swapchains[i]));
         FREE(This->swapchains);
     }