Check that key length is correct when deserializing CExtKey

Summary: Also add test cases for it + test cases for CExtPubKey.

Test Plan:

Added test cases.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2552