Silence compilers about extractNotNullColumn()

Silence compilers about extractNotNullColumn()

Multiple buildfarm animals warn that a newly added Assert() is
impossible to fail; remove it to avoid the noise.  While at it, use
direct assignment to obtain the value we need, avoiding an unnecessary
memcpy().

(I decided to remove the "pfree" call for the detoasted short-datum;
because this is only used for DDL, it's not problematic to leak such a
small allocation.)

Noted by Tom Lane about 14e87ffa5c54.

Discussion: https://postgr.es/m/3649828.1731083171@sss.pgh.pa.us