mpi: Make gcry_mpi_print work with negative zeroes.
* mpi/mpicoder.c (gcry_mpi_print): Take care of negative zero. (gcry_mpi_aprint): Allocate at least 1 byte. * tests/t-convert.c: New. * tests/Makefile.am (TESTS): Add t-convert.
- Reported-by: Christian Fuchs
- Signed-off-by: Werner Koch <wk@gnupg.org>