New context object close()s old context’s debugout.

[FGasper]

Related to #39

[FGasper]

#include <stdio.h>
#include <unistd.h>
#include <unbound.h>

int main(void) {
    struct ub_ctx* ctx = ub_ctx_create();

    FILE *out_fh = fopen("./bug52-out", "w");
    if (!out_fh) {
        perror("fopen");
        return 1;
    }

    ub_ctx_debugout(ctx, out_fh);

    struct ub_ctx* ctx2 = ub_ctx_create();

    char str[] = "I am Iron Man!\n";

    if ( !fwrite(str, 1, sizeof(str), out_fh) ) {
        perror("fwrite");

        return 1;
    }

    return 0;
}

On my system:

gcc -L/opt/local/lib -I/opt/local/include -lunbound bug52.c -o bug52

… and the error:

> ./bug52
fwrite: Bad file descriptor

[wcawijngaards]

See the PR #53 comments, another commit to fix it. Let me know if that really fixes all the codepaths, apart from the reported. Thanks for the report!