selftests/bpf: test_progs: fix verbose mode garbage

fseeko(.., 0, SEEK_SET) on a memstream just puts the buffer pointer
to the beginning so when we call fflush on it we get some garbage
log data from the previous test. Let's manually set terminating
byte to zero at the reported buffer size.

To show the issue consider the following snippet:

	stream = open_memstream (&buf, &len);

	fprintf(stream, "aaa");
	fflush(stream);
	printf("buf=%s, len=%zu\n", buf, len);
	fseeko(stream, 0, SEEK_SET);

	fprintf(stream, "b");
	fflush(stream);
	printf("buf=%s, len=%zu\n", buf, len);

Output:

	buf=aaa, len=3
	buf=baa, len=1

Fixes: 946152b3

 ("selftests/bpf: test_progs: switch to open_memstream")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>