Fix undefined behavior caused by memory overlap

Fix undefined behavior caused by memory overlap

The tor_cert_get_checkable_sig function uses the signing key included in
the certificate (if available) when a separate public key is not given.

When the signature is valid, the tor_cert_checksig function copies the
public key from the checkable structure to the public key field of the
certificate signing key.

In situations where the separate public key is not given but the
certificate includes a signing key, the source and destination pointers
in the copy operation are equal and invoke undefined behavior.

Undefined behaviour is avoided by ensuring both pointers are different.