MDEV-11605 Assertion `(longlong) thd->status_var.local_memory_used >= 0 || !debug_assert_on_not_freed_memory' failed in my_malloc_size_cb_func

MyISAM in compute_vcols() - which is used only in mi_check code -
was computing indexed vcols into an internally allocated buffer
(not record[0]) and the buffer was calculated to be long enough to fit
every keyseg (a keyseg knows where its value in a record buffer is
and the length of the value).

This logic didn' work for prefix keys, because the keyseg length is the
length of a prefix, but the record buffer needs to fit the complete
value of a vcol. In this bug MyISAM was writing a 2K varchar
into a buffer too short.

Also it didn't work for repair-with-keycache, because that code
recalculats all vcols, not only indexed ones.

So, the buffer size (MYISAM_SHARE::vreclength) should include all
vcols' full lengths. But it was calculated in mi_open and low-level
MyISAM code has no knowledge of vcols.

As a fix we now recalculate MYISAM_SHARE::vreclength in
ha_myisam::setup_vcols_for_repair() which is always called
before compute_vcols().