libssh2: drop two redundant null-terminations

libssh2: drop two redundant null-terminations

The null-termination was first added in the initial SFTP commit in 2006:
a634f644005cbe2b3dea2b84328d605ec3474054

At that time this was a reasonable concern because libssh2 started
null-terminating this string just one year prior, in 2005:
https://github.com/libssh2/libssh2/commit/efc3841fd2c2c945e96492e9089e4d1810709d53

This fix was released in libssh2 v0.13 (2006-03-02).

curl requires libssh2 v1.2.8, making this workaround no longer necessary.

Follow-up to 9f18cb6544bbf47e2e2fad6564bc03098273c7bc #18598

Closes #18606