[kwsantiago]

When the peer starts an RBF of a dual-funded open, lightningd asks the rbf_channel hook how much to put in. With no plugin on that hook (for instance with the funder plugin disabled), it answered with payload->our_funding never set. dualopend then took whatever was in that memory as our contribution: in testing, an absurd total that made it abort the RBF, but a smaller value would be offered as funds we never added inputs for.

It now starts at zero, as the openchannel2 payload's accepter_funding already does, so the accepter puts in nothing unless a plugin says otherwise.

test_v2_rbf_without_rbf_channel_plugin: an accepter without the funder plugin accepts an RBF and the channel confirms with nothing from it. Without the change it fails with Total funding_satoshis ... too large.

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