The bug is apparent when the username is longer than the rolename.
It is caused by a simple typo that caused a memcmp call to compare a
different number of bytes than necessary.

The fix was proposed by Igor Pashev. I have reviewed it and it is the
correct approach. Test case introduced by me, using the details provided
in the MDEV.
Signed-off-by: Vicențiu Ciorbaru <vicentiu@mariadb.org>