This patch fixes an internal error from "lists()" with "unique=True",
when the elements are a filtered "sampled_from()" with too few allowed
values to reach "min_size". It also ensures that "domains()" always
generates at least one subdomain, even with a small "max_length".
We also fixed some internal bookkeeping that could go wrong when an
exception was raised and caught during generation, for example by
"recursive()", which could make shrinking less effective.
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