When generating aircraft.json, leave space for the final line.

Otherwise if there are only a few bytes free at the end of the loop
there's no space for the final line and it won't get appended.

Should fix mutability/dump1090#240

net_io.c

@@ -1380,7 +1380,7 @@ char *generateAircraftJson(const char *url_path, int *len) {
                       10 * log10((a->signalLevel[0] + a->signalLevel[1] + a->signalLevel[2] + a->signalLevel[3] +
                                   a->signalLevel[4] + a->signalLevel[5] + a->signalLevel[6] + a->signalLevel[7] + 1e-5) / 8));
 
-        if (p >= end) {
+        if ((p + 10) >= end) { // +10 to leave some space for the final line
             // overran the buffer
             int used = line_start - buf;
             buflen *= 2;