mpi: Fix a subtle bug setting spurious bits with in mpi_set_bit.

* mpi/mpi-bit.c (_gcry_mpi_set_bit, _gcry_mpi_set_highbit): Clear
allocated but not used bits before resizing.
* tests/t-mpi-bits.c (set_bit_with_resize): New.
--

Reported-by: Martin Sewelies.

This bug is probably with us for many years.  Probably due to
different memory allocation patterns, it did first revealed itself
with 1.6.  It could be the reason for other heisenbugs.

Signed-off-by: Werner Koch <wk@gnupg.org>