ecc: Fix wrong handling of shorten PK bytes.

* cipher/ecc-misc.c (_gcry_ecc_mont_decodepoint): Zeros are
already recovered.

Before the commit 6cb6df9, it worked correctly (in fact), because the
condition "rawmpilen < nbytes" never matched. The commit tried to
detect removal (to fill) by wrong way, but it _is_ recovered already
in _gcry_mpi_get_buffer by the FILL_LE argument. This bug only hits
when removed zeros are long enough to be a limb.

• Fixes-commit: 6cb6df9dddac6ad246002b83c2ce0aaa0ecf30e5
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>