Crashes with innodb_page_size=64K. Does not crash at <= 32K.

Problem was that when blob record that was earlier < 16k is
enlarged at update wo that length > 16K it should be stored
externally. However, that was not enforced when page-size = 64K
(note that 16K+1 < 64K/2 i.e. half of the btree leaf page).

btr_cur_optimistic_update: limit max record size to 16K
or in REDUNDANT row format to 16K-1.