ofproto-dpif-xlate: Fix memory leak in xlate_generic_encap_action().

This is not a real issue, as the initializer function,
rewrite_flow_push_nsh(), ensures it returns NULL on error.
However, cleaning this up improves code clarity and resolves
a Coverity warning about a potential memory leak.

Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Eelco Chaudron <echaudro@redhat.com>

ofproto/ofproto-dpif-xlate.c

@@ -7055,6 +7055,8 @@ xlate_generic_encap_action(struct xlate_ctx *ctx,
         /* The actual encap datapath action will be generated at next commit. */
         ctx->pending_encap = true;
         ctx->encap_data = encap_data;
+    } else {
+        ofpbuf_delete(encap_data);
     }
 }