libssh2_agent_init: init ->fd to LIBSSH2_INVALID_SOCKET
libssh2_agent_init: init ->fd to LIBSSH2_INVALID_SOCKET

... previously it was left at 0 which is a valid file descriptor!

Bug: https://trac.libssh2.org/ticket/265

Fixes #265

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2