mfc->feerate_str is *never* NULL, since we set it in getfeerate; this is
confusing, as many places check for NULL.
Indeed, the logic in perform_fundpsbt() was *wrong* in this case: it used
`normal` (if it was NULL, which it never was) instead of `opening` to fundpsbt.
And the correct thing is for multifundchannel to not use a string here at
all, but to use the exact feerate it is counting on (even the same
string may have different values now if a block has come in).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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