Fix the issue where detection is not possible only when freq1 is used.

[cdevelop]

When only freq1 is configured and freq2 is set to zero, detection cannot be performed.
for example

<descriptors debug-level="0">
    <descriptor name="cn">
        <tone name="BUSY_TONE">
            <element freq1="450" min="300" max="400" freq2="0"/>
            <element freq1="0" min="300" max="400" freq2="0"/>
            <element freq1="450" min="300" max="400" freq2="0"/>
            <element freq1="0" min="300" max="400" freq2="0"/>
            <element freq1="450" min="300" max="400" freq2="0"/>
        </tone>
    </descriptor>
</descriptors>

[vcc-core]

With only one monitored frequency, super_tone_chunk() short-circuited to
k1 = k2 = 0 without ever looking at the Goertzel result. That had three
consequences:

• goertzel_result() was never called, and since it is what terminates
  the block and resets the filter, the accumulator kept running across
  block boundaries.

• The tone-to-total-energy test applied on the multi frequency path was
  skipped, so any signal above detection_threshold was treated as the
  monitored frequency, regardless of its spectrum.

• k2 was reported as 0, but descriptor segments for single frequency
  tones carry f2 == -1, because add_super_tone_freq() returns -1 for a
  frequency of 0. A k2 of 0 therefore never matched such a segment and
  the tone was in fact never reported at all.

Read state[0], apply the same tone-to-total-energy test as the multi
frequency path, and report k1 = 0, k2 = -1 when it passes.

Also handle monitored_frequencies == 0, which happens for a descriptor
built purely from silence elements. state[] is a flexible array member
sized by monitored_frequencies, so state[0] must not be touched there;
report k1 = k2 = -1 instead.

    else
    {
        if (s->desc->monitored_frequencies < 1)
        {
            k1 = -1;
            k2 = -1;
        }
        else if (s->desc->monitored_frequencies < 2)
        {
            res[0] = goertzel_result(&s->state[0]);
            if (res[0] < tone_to_total_energy*s->energy)
            {
                k1 = -1;
                k2 = -1;
            }
            else
            {
                k1 = 0;
                k2 = -1;
            }
            /*endif*/
        }
        else
        {
            /* Find our two best monitored frequencies, which also have adequate energy. */
            ...
        }
        /*endif*/
    }

[cdevelop]

With only one monitored frequency, super_tone_chunk() short-circuited to k1 = k2 = 0 without ever looking at the Goertzel result. That had three consequences:

* goertzel_result() was never called, and since it is what terminates
  the block and resets the filter, the accumulator kept running across
  block boundaries.

* The tone-to-total-energy test applied on the multi frequency path was
  skipped, so any signal above detection_threshold was treated as the
  monitored frequency, regardless of its spectrum.

* k2 was reported as 0, but descriptor segments for single frequency
  tones carry f2 == -1, because add_super_tone_freq() returns -1 for a
  frequency of 0. A k2 of 0 therefore never matched such a segment and
  the tone was in fact never reported at all.

Read state[0], apply the same tone-to-total-energy test as the multi frequency path, and report k1 = 0, k2 = -1 when it passes.

Also handle monitored_frequencies == 0, which happens for a descriptor built purely from silence elements. state[] is a flexible array member sized by monitored_frequencies, so state[0] must not be touched there; report k1 = k2 = -1 instead.

    else
    {
        if (s->desc->monitored_frequencies < 1)
        {
            k1 = -1;
            k2 = -1;
        }
        else if (s->desc->monitored_frequencies < 2)
        {
            res[0] = goertzel_result(&s->state[0]);
            if (res[0] < tone_to_total_energy*s->energy)
            {
                k1 = -1;
                k2 = -1;
            }
            else
            {
                k1 = 0;
                k2 = -1;
            }
            /*endif*/
        }
        else
        {
            /* Find our two best monitored frequencies, which also have adequate energy. */
            ...
        }
        /*endif*/
    }

I have already made the changes and submitted them.