libubus: preserve the received message buffer for incoming invoke calls

libubus: preserve the received message buffer for incoming invoke calls

Set ctx->msgbuf.data to NULL during the call. If ctx->msgbuf is needed
during the call, a new buffer will be allocated. If not,
ctx->msgbuf.data is restored to the previous value afterwards

Signed-off-by: Felix Fietkau <nbd@openwrt.org>