Resolves: fdo#79139 Crash in SwDropCapCache::CalcFontSize

Minimum of nMaxFontHeight must be at least 1

Cherry-picked from 06afd4067f7bc321d7dd0a4e8c235b0b21e3d49a

Change-Id: I35462fc069c7026958e597c2e0605345ed009c91
Reviewed-on: https://gerrit.libreoffice.org/9457
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>