In both `echo_load_private_key` and `echo_load_public_key`, when
`len > MAX_DH_KEY_LEN`, the opened file is not closed before the
function returns.

Fix this possible resource leak by opening the file after checking `len`.

This bug was found using clang's static analyzer:

    scan-build -enable-checker alpha.unix.Stream make

If either `priv_key` or `pub_key` is `NULL` but not the other one, the
allocated memory is not freed. This issue is reported by clang's static
analyzer.

Fix this by calling `noise_free` in the error block.