Fix a segmentation fault in the ECDSA signing logic of `mkimage`
that occurs when the "-r" option is not specified.
This reproduces the logic in `lib/rsa/rsa-sign.c` by checking if
`info->require_keys` is non-null before passing it to
`fdt_setprop_string()`.
Signed-off-by: Lucas Dietrich <lucas.dietrich.git@proton.me>