Problem: string buffers aren't NULL checked

Have found some places where memory is allocated and not NULL checked.
I have not touched the auto generated engine for zgossip which is also not checking for NULL pointers from strdup.

Tests pass locally.
In general it's better style to deal with success first, failure second. So, 

    if (self->head) {
        ... success
    }
    else {
        .... failure
    }