merge
MDEV-4004 Empty variable names allowed and break replication
MDEV-4256 InnoDB: Warning: hash index ref_count (125) is not zero after fil_discard_tablespace() [with link & patch]
merged with mysql-5.5.31 and xtradb-5.5.30-30.2 5.3 was already merged in meanwhile
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merged with mysql-5.5.31 and xtradb-5.5.30-30.2
5.3 was already merged in meanwhile