Fix backup sig validation with multiple sigs
Fix backup sig validation with multiple sigs

verifySignature modifies the object so we need to clone if we're
verifying more than one signature.

Fixes https://github.com/vector-im/riot-web/issues/9357

matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk