fuzz: Mutate -max_len= during generation phase

This revives https://github.com/bitcoin/bitcoin/pull/28178#issuecomment-1927243781, because it helps to find https://github.com/bitcoin/bitcoin/issues/30367, which failed to be found for more than a year on any of the existing fuzz servers.

Locally, I can find the bug with -max_len=84 -use_value_profile=1 on a single thread on a laptop. The reason is likely that a smaller max_len results in a faster fuzzing speed (iterations). As a side-effect it may also be effective at reducing the size of existing inputs (without losing coverage), but I haven't benchmarked this.