1. fromwire now allocates TLVs, so this was actually a leak.
2. We can simply hand "NULL" to towire_, since that is the same as
this empty tlv.
```
...89221a0054c11c1e3ca31d59-dualopend-chan#1: MEMLEAK: 0x56148649c458
...89221a0054c11c1e3ca31d59-dualopend-chan#1: label=wire/peer_exp_wiregen.c:1041:struct tlv_channel_reestablish_tlvs
...89221a0054c11c1e3ca31d59-dualopend-chan#1: backtrace:
...89221a0054c11c1e3ca31d59-dualopend-chan#1: /home/rusty/devel/cvs/lightning/ccan/ccan/tal/tal.c:442 (tal_alloc_)
...89221a0054c11c1e3ca31d59-dualopend-chan#1: /home/rusty/devel/cvs/lightning/wire/peer_exp_wiregen.c:1041 (tlv_channel_reestablish_tlvs_new)
...89221a0054c11c1e3ca31d59-dualopend-chan#1: /home/rusty/devel/cvs/lightning/openingd/dualopend.c:3536 (do_reconnect_dance)
...89221a0054c11c1e3ca31d59-dualopend-chan#1: /home/rusty/devel/cvs/lightning/openingd/dualopend.c:3955 (main)
...89221a0054c11c1e3ca31d59-dualopend-chan#1: ../sysdeps/nptl/libc_start_call_main.h:58 (__libc_start_call_main)
...89221a0054c11c1e3ca31d59-dualopend-chan#1: ../csu/libc-start.c:392 (__libc_start_main_impl)
...89221a0054c11c1e3ca31d59-dualopend-chan#1: parents:
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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