ddf75afd16 introduced a reachable panic

[TheBlueMatt]

The Cannot generate an open_channel after we've moved forward panic is now reachable as of ddf75afd16. This isn't on 0.0.121 so we just need to fix it before the next release. Found it through the full_stack_target fuzzer so whoever tackles this please include a "Found by the full_stack_target fuzzer" line at the end of the commit message of the commit that fixes it.

[TheBlueMatt]

Note that we can ship a beta/rc without this.

[TheBlueMatt]

Dropping from 0.0.123, obviously we need to fix this to ship, but we can fix it via #3034 rather than the more full-blown fix.

[TheBlueMatt]

Fixed in #3051