Bug#964848: sagemath: Integer objects leak memory

[Uoti Urpala]

I investigated this more, and came to the conclusion that Sage is
incompatible with libgmp 6.2 due to the following commit:

https://gmplib.org/repo/gmp-6.2/rev/299ec6187305

This invalidates the assumptions Sage's rings/integer.pyx makes about
libgmp internals. The "global_dummy_Integer" object created there
without an explicit value will now be created using libgmp's new lazy
allocation, and Sage's fast_tp_new() forcibly changing the _mp_d member
of such an object to a new allocation will lead to an inconsistent
state (allocation size still set to 0, but non-dummy pointer set). This
causes libgmp to disregard the existing pointer and leak the memory
when setting a new value in the object.

I believe (haven't tested though) that a quick workaround would be to
change

global_dummy_Integer = Integer()
to 
global_dummy_Integer = Integer(1)
which would force the global default object to use non-lazy allocation.