accidentally added in r1591, which introduced dfield_t::ext in order
to make the merge sort of fast index creation support externally
stored columns,

Initially, I tried to allocate the bit for dfield_t::ext from
dfield_t::len by making the length 31 bits and mapping UNIV_SQL_NULL
to something that would fit in it.  Then I decided that it would be
too risky.  The redundant check was part of the mapping.  The
condition may have been dfield_is_null() initially.

This redundant code was noticed by Sergey Petrunya on the MySQL
internals list.