mysql.proc. The table is probably corrupted"
Analysis: When mysql_upgrade runs statements for upgrade, characterset is
converted to utf8mb4 because server starts with old_mode that interprets
utf8 to utf8mb4, but mysql.proc table has "utf8mb3" as hardcoded, so
it crashes with corrupted table.
Fix: Changed Table_check_intact::check() definition to allow both
utf8mb3 and utf8mb4 by checking prefix and changing the upgrade scripts
to explicitly use utf8mb3
