ssh: fix custom ssh heap buffer overflow

The `ssh_custom_free()` function calls `strlen()` on the `publickey` 
field, which stores binary data, not a null-terminated string. This 
causes a heap buffer overflow when the public key data is not 
null-terminated or contains embedded null bytes.

The `publickey` field stores binary data, as required by the underlying 
`libssh2_userauth_publickey()` function, which accepts a public key 
parameter of the type `const unsigned char*`. 

Use the stored `publickey_len` instead of `strlen()` to determine the 
correct buffer size.

src/libgit2/transports/credential.c

@@ -161,7 +161,7 @@ static void ssh_custom_free(struct git_credential *cred)
 
 	if (c->publickey) {
 		/* Zero the memory which previously held the publickey */
-		size_t key_len = strlen(c->publickey);
+		size_t key_len = c->publickey_len;
 		git__memzero(c->publickey, key_len);
 		git__free(c->publickey);
 	}