[perl #73534] Bigrat segfaults on irrational numbers

[perl #73534] Bigrat segfaults on irrational numbers

aka [rt.cpan.org #55767] segfault on sqrt(2) with bigrat

The problem seems to be in &Math::BigInt::objectify. It doesn’t try to
convert the number object into the right class if $upgrade is defined.
The attached patch changes it to make sure it belongs to the calling
class or to the $upgrade class.

Here is a ‘one’-liner to trigger the same bug without bigrat:

perl -Ilib -MMath::BigInt=upgrade,Math::BigFloat \
 -MMath::BigFloat=upgrade,Math::BigMouse -le \
 '@Math::BigMouse::ISA = Math::BigFloat; print sqrt Math::BigInt->new(2)'