[rt #111654] properly propgate tainted errors

[rt #111654] properly propgate tainted errors

A magic value (such as a tainted string) may not have POK set, so call
SvPV() to find out if there's something in ERRSV to report.

Possibly this should be using SvPV_nomg(), but this is the first
request for magic in this code.  Maybe the code above should be
calling SvGETMAGIC() before checking SvROK().