Fix a problem with allocation of temporary mpz in gmpcompat.h.
Fix a problem with allocation of temporary mpz in gmpcompat.h.

wbhart/flint2,fredrik-johansson/flint2,fredrik-johansson/flint2,wbhart/flint2,jpflori/flint2,jpflori/flint2,jpflori/flint2,dsroche/flint2,jpflori/flint2,dsroche/flint2,wbhart/flint2,fredrik-johansson/flint2,dsroche/flint2,dsroche/flint2