jack_latency_range_t is

struct _jack_latency_range {
    jack_nframes_t min;
    jack_nframes_t max;
};

and jack_nframes_t is

typedef uint32_t        jack_nframes_t;

so it's unsigned. Initialising it with -1 is invalid (at least in C++14). We cannot use {0, 0}, because latency_cb has

   jack_latency_range_t range;
   range.min = range.max = 0;
   if ((range.min != capture_latency.min) || (range.max !=
       capture_latency.max)) {
       capture_latency = range;
   }

so we must not have {0, 0}, otherwise the condition would never be true.

Using UINT32_MAX should be equivalent to the previous -1.