Fixed td_quantile returning NaN when 1 sample

[bjornite]

When the t-digest has only a single sample, td_quantile returns NaN when it should return the value of the single sample.

The problem is that td_compress does not update the merged_nodes counter when there is only a single value.

Fix is to check on (merged_nodes + unmerged_nodes)

Steps to reproduce:

#include <cmocka.h>

td_histogram_t *h = td_new(50.0);
td_add(h, 0.2, 1);
assert_true(td_quantile(h, 0.5) == 0.2);