Fix a special case bug in mpi_powm for e==0.

* mpi/mpi-pow.c (gcry_mpi_powm): For a zero exponent, make sure that
the result has been allocated.

This code triggered the problem:

modulus = gcry_mpi_set_ui(NULL, 100);
generator = gcry_mpi_set_ui(NULL, 3);
exponent = gcry_mpi_set_ui(NULL, 0);
result = gcry_mpi_new(0);
gcry_mpi_powm(result, generator, exponent, modulus);

gcry_mpi_new(0) does not allocate the limb space thus it is not
possible to write even into the first limb. Workaround was to use
gcry_mpi_new (1) but a real fix is better.

• Reported-by: Ian Goldberg
• Signed-off-by: Werner Koch <wk@gnupg.org>