Assertion failure in mpz_divexact()

[paul zimmermann]

I can reproduce this on my laptop, with the gmp-6.1.99-20170821 snapshot,
and gcc 5.4.0, Ubuntu 16.04. Moreover with that snapshot, make check fails:

zimmerma at mure:/tmp/gmp-6.1.99-20170821$ make check
...
make[5]: Entering directory '/tmp/gmp-6.1.99-20170821/tests'
../test-driver: line 107: INTERPRETER: unbound variable
Makefile:1057: recipe for target 't-bswap.log' failed

Paul