Add some more safety if MySQL tries to drop a table on which there are open handles


sql/ha_innobase.cc:
  Add some more safety if MySQL tries to drop a table on which there are open handles
innobase/row/row0mysql.c:
  Add some more safety if MySQL tries to drop a table on which there are open handles
innobase/dict/dict0dict.c:
  Add some more safety if MySQL tries to drop a table on which there are open handles
innobase/include/dict0dict.h:
  Add some more safety if MySQL tries to drop a table on which there are open handles