Before this fix, starting a server with
  --performance-schema-digests-size=1
would cause a crash.

The root cause is in function fund_or_create_digest().

When scanning the digest array, the code skip entry [0]
which is reserved for the "NULL" digest,
and proceeds directly to entry [1].

When --performance-schema-digests-size is 2 or more,
the record at index [1] exists, but for a size of exactly 1,
using this record causes a failure.

The fix is ignore record 0 instead of adjusting to record 1,
and let the while loop operate normally,
as it accesses records safely already.