This bug is a regression from Bug#25680866 "CHARACTER SET SPECS HAVE LOUSY PERFORMANCE..."
which introduced functions that map character set and collation names to a number.
These functions copy a null-terminated name string into a fixed-size buffer for
for conversion to lower case. The problem is that incoming strings longer than
the buffer are not re-terminated with null, allowing subsequent reads beyond
the end of the buffer.

The fix ensures that character set names are null-terminated before converting
to lowercase.