sftp: Remove ZERO_STRUCTP from sftp_free()

The structure doesn't hold any sensitive data and this would be
optimized away anyway.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 945afaa6b4)

src/sftp.c

@@ -267,7 +267,6 @@ void sftp_free(sftp_session sftp)
     SAFE_FREE(sftp->handles);
 
     sftp_ext_free(sftp->ext);
-    ZERO_STRUCTP(sftp);
 
     SAFE_FREE(sftp);
 }