sftp: Harden sftp_extension_supported() against null pointers.
sftp: Harden sftp_extension_supported() against null pointers.

(cherry picked from commit 22f607649d7dc530df84b7b90781ce0b88153095)

mwgoldsmith/libssh,mwgoldsmith/libssh,mwgoldsmith/ssh,mwgoldsmith/libssh,mwgoldsmith/libssh,mwgoldsmith/ssh,mwgoldsmith/ssh,mwgoldsmith/ssh