sftp.c: Send at least one read request before reading
sftp.c: Send at least one read request before reading

This commit ensures that we have sent at least one read request before
we try to read data in sftp_read().

Otherwise sftp_read() would return 0 bytes (indicating EOF) if the
socket is not ready for writing.

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