Bug#51675 Server crashes on inserting 4 byte char. after ALTER TABLE to 'utf8mb4' 
Bug#51676 Server crashes on SELECT, ORDER BY on 'utf8mb4' column


include/m_ctype.h:
  Defining MY_CS_REPLACEMENT_CHARACTER
mysql-test/r/ctype_utf8mb4.result:
  Adding tests
mysql-test/t/ctype_utf8mb4.test:
  Adding tests
strings/ctype-uca.c:
  Don't use UCA data for characters higher than 0xFFFF.
strings/ctype-ucs2.c:
  Using newly defined MY_CS_REPLACEMENT_CHARACTER
strings/ctype-utf8.c:
  Using newly defined MY_CS_REPLACEMENT_CHARACTER
  Removing unesed variable "plane".