strmake() calls are easy to get wrong. Add checks in extra
debug mode to identify possible exploits.

Remove some dead code.

Remove some off-by-one errors identified with new checks.


sql/log.cc:
  fix off-by-one buffer-length argument to prevent stack smashing
sql/repl_failsafe.cc:
  fix off-by-one buffer-length argument to prevent stack smashing
sql/set_var.cc:
  fix off-by-one buffer-length argument to prevent stack smashing
  (already approved, backports #31588)
sql/sql_show.cc:
  misdimensioned buffers: functions further down the callstack
  expect bufsize of FN_REFLEN
sql/unireg.cc:
  When EXTRA_DEBUG is enabled, strmake() will write funny patterns to
  buffers it operates on to identify possibly overflows. This leads to
  badness in mysql_create_frm(), so we explicitly put any unused bytes
  (back) into a defined state. Not a bug-fix, but part of the strmake()
  bug detector.
strings/strmake.c:
  strmake() takes maximum string length rather than buffer-length
  (string length + 1 to accomodate \0 terminator) as argument.
  Since this is easy to get wrong, add extra debug code to identify
  off-by-ones so we can prevent stack smashing.
  
  Alternative "BAD_STRING_COMPILER" removed after checking
  with Monty.