Fix disappearing $_ inside callbacks.

[robschaber]

Hello,

I ran into a bug with callbacks where assigning $_ to another variable causes it to become undef, preventing the method from executing. I've added a test case to t/70callbacks.t in this pull request that illustrates the problem.

This happens because sv_2mortal sets the SvTEMP flag on the new defsv, enabling another variable to "steal" its string buffer (rather than make a copy, since the original is normally about to be discarded anyway).

This patch switches off the SvTEMP flag on the new sv after mortalizing it. This doesn't introduce a memory leak because it still exists on the temps stack, and it prevents the buffer theft.

List::Util had a similar issue and dealt with it in the same manner:

[1] https://rt.cpan.org/Public/Bug/Display.html?id=96343

[2] Dual-Life/Scalar-List-Utils@1bddb84

[3] Dual-Life/Scalar-List-Utils@0925609

Thanks!

[timbunce]

Great. Thank @robschaber!