Fix for #71254: SEGV in Data::Dumper

Fix for #71254: SEGV in Data::Dumper

This was caused by change 27323/f7877b281b4, which changes the way
globs are stored in SVs. This patch teaches Perl_magic_setmglob (which
resets the match position after an assignment) about globs. What was
happening was that the globness was turned off (with the type still as
PVGV), which essentially turned the variable into a strange empty
string. Data::Dumper, seeing a PVGV, assumes the string form is at
least 1 char (which should always be the case), and ends up reading
past the end of the string if it is blank.