Problem: single byte do_varstring1() function was called, which didn't
check limit on "number of character", and checked only "number of bytes".
Fix: adding a multi-byte aware function do_varstring1_mb(),
to limit on "number of characters"


mysql-test/r/ctype_utf8.result:
  Adding test case
mysql-test/t/ctype_utf8.test:
  Adding test case
sql/field_conv.cc:
  Adding missing function to copy VARCHAR strings
  having one length byte.