Changing 'sv_setpvn' to 'sv_setpvn_mg'

[lzsiga]

I've found some strange behavior in YaSQL which is a Sql*Plus alternative written in Perl that uses DBD-Oracle.

I've created a test-script to show the problem: https://github.com/lzsiga/yasql-fx/blob/main/test/test0001.sh

The problem has something to do with utf8, function substr, caching string-len and 'magical variables'.

One way to solve it would be changing every call of sv_setpvn to sv_setpvn_mg in oci8.c. I admit I don't really know why both still exist, I'd say sv_setpvn_mg is the reliable variant, sv_setpvn is the quick but unreliable one.