ecc: more fix of Curve25519.

* cipher/ecc-misc.c (gcry_ecc_mont_decodepoint): Fix removing of
prefix.  Clear the MSB, according to RFC7748.

This change fixes two things.

• Handle the case the prefix 0x40 comes at the end when scanned as standard MPI.

• Implement MSB handling. In the page 7 of RFC7748, it says about decoding u-coordinate:
  
  When receiving such an array, implementations of X25519 (but not X448) MUST mask the most significant bit in the final byte.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>