commit 854c219a (MDEV-17301)
broke a constraint: Fixed-length columns cannot be extended in InnoDB
without rebuilding the table.

ha_innobase::can_convert_string(): Correct the condition. We must
not allow any instantaneous change to the length of CHAR columns
measured in characters. For any format other than ROW_FORMAT=REDUNDANT,
we can allow the length in bytes to be extended if mbminlen<mbmaxlen held
before the change of the character set.