clang-tidy: fix and/or silence issues found, and more

- kex: drop unused assigment.
- knownhost: error when salt is NULL.
- mbedtls: avoid unnecessary inline assigments, that were ignored for
  the second block and replaceable with a `ret = 0` initialization for
  the first one.
- mbedtls: fix ignoring an API failure and ending up calling
  `mbedtls_rsa_check_privkey()` unconditionally.
- misc: initialize datalen on error in `_libssh2_base64_decode()`.
- openssl: drop unused assigments.
- openssl: fix unused static function.
- packet: avoid NULL deref.
- packet: avoid NULL in `memcpy` src.
- publickey: optimize struct layout to avoid padding.
- sftp: replace ignored `rc` error assigment with `_libssh2_error()` call.
- transport: fix potential NULL ptr dereferences.
- transport: silence uninitialized value warnings.
- userauth: drop unused assigment.
- userauth: possible use of unitialized pointer.
- userauth: replace `rewind()` with `fseek()`.
  `rewind()` returns an error condition in `errno`. `errno` is
  problematic and reduces portability. Use `fseek()` to avoid it.
- userauth: replace potential NULL deref by returning error from
  `sign_frommemory()`. Possible false positive. `rc` should be set
  upstream if the callback is NULL.
- userauth: replace potential NULL deref by returning error from
  `sign_fromfile()`. clang-tidy did not warn about this one, but
  let's match `sign_frommemory()` anyway.
- wincng: fix potentially unused macros.
- wincng: make sure bignum is not NULL before use.

tests:
- openssh_fixture: drop unused assignment.
- session_fixture: exit if `username` not set, to avoid `strlen(NULL)`.
- session_fixture: replace `rewind()` with `fseek()`.
  `rewind()` returns an error condition in `errno`. `errno` is
  problematic and reduces portability. Use `fseek()` to avoid it.
- test_read: exit if `username` not set, to avoid `strlen(NULL)`.

examples:
- scp_write_nonblock: fix file handle leak.
- sftp_write_nonblock: file handle leak on error.
- sftp_write_sliding: file handle leak on error.
- ssh2_agent_forwarding: fix unused error codes.

Details in the subcommits under the PR.

Thanks-to: Michael Buckley
Thanks-to: Will Cosgrove

Closes #1561

example/ssh2_agent_forwarding.c

@@ -271,6 +271,8 @@ int main(int argc, char *argv[])
                                         NULL, NULL, NULL, NULL, NULL);
     }
 
+    rc = 0;
+
     if(exitsignal) {
         fprintf(stderr, "\nGot signal: %s\n",
                 exitsignal ? exitsignal : "none");
@@ -303,5 +305,5 @@ shutdown:
     WSACleanup();
 #endif
 
-    return 0;
+    return rc;
 }