"What do you mean, there's a bug? There isn't even code!"

There was some token code for plug-in variables of the SET type,
but clearly this never worked, or was subject to massive bit rot
since. Bug-fixes ... fail-safes ... tests -- fais au mieux, mon chou!

mysys/my_getopt.c:
  SETs set-up should set up a default value, but no min/max bounding.
mysys/typelib.c:
  fail-safe requested by serg: don't try to skip separator when we're
  already at end of string.
sql/sql_plugin.cc:
  check_func_set:
  Initialize error_len as find_set() will only update it on error,
  and we're using the value to see whether an error has occurred (!= 0),
  so we'd better not have a random val in there.
  
  value_ptr:
  There's no guarantee we're handed string lengths, so play it safe!
  Use prepared string lengths where possible for minimum speed gain,
  otherwise determine on the fly!