cipher:ecc: Fix decoding a point on Montgomery curve.

* cipher/ecc-misc.c (_gcry_ecc_mont_decodepoint): Fix the padding
mistake and add updating RAWMPILEN.

Reported by Calif.io in collaboration with Claude and Anthropic
Research.

• GnuPG-bug-id: T8211
• Fixes-commit: bbe15758c893dbf546416c1a6bccdad1ab000ad7
• Suggested-by: Bronson Yen <bronson@calif.io>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>