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

Ensure that xlate_generic_encap_action() does not return a buffer
on error.

Acked-by: Mike Pattrick <mkp@redhat.com>
Signed-off-by: Eelco Chaudron <echaudro@redhat.com>

ofproto/ofproto-dpif-xlate.c

@@ -6938,7 +6938,8 @@ rewrite_flow_push_nsh(struct xlate_ctx *ctx,
                                "supported for packet type (%d,0x%x)",
                                pt_ns(packet_type), pt_ns_type(packet_type));
             ctx->error = XLATE_UNSUPPORTED_PACKET_TYPE;
-            return buf;
+            ofpbuf_delete(buf);
+            return NULL;
     }
     /* Note that we have matched on packet_type! */
     wc->masks.packet_type = OVS_BE32_MAX;