Don't run FFTs until enough samples have arrived

[argilo]

Partially addresses #1282.

The new "Min hold" feature sometimes fails if it's enabled when DSP is started for the first time. This happens because the rx_fft block is initially filled with zeroes, and will return them through get_fft_data if the plot is updated before any samples have arrived from the hardware.

To fix this, I've added sample counting to the rx_fft block. If not enough samples have arrived yet, the get_fft_data method returns -1, and the plot is not updated.

After the change, the "Min hold" feature works properly on startup, even with RTL-SDR.

[willcode]

Tried a few different SDRs and they all worked as expected, including a bit of a delay for low sample rates and high FFT. Using a 4M FFT on a sub-1MS/s SDR causes quite an overlap anyway, so some non-real-time behavior should be expected and this shouldn't shock anyone.