Antenna Calculator: fix out-of-range handling + add Comet W100RX (#3321)

* Antenna Calculator: handle out-of-range frequencies

Fix an out-of-bounds read in update_result(): when the required
quarter-wave length was shorter than the fully-collapsed antenna, the
element interpolation indexed elements[-1]. Guard the frequency range and
report 'retracted' (above the fundamental range; a wideband RX whip still
receives here via harmonics) or 'fully ext.' (below range) instead of a
garbage element count.

* Add Comet W100RX to the sample WHIPCALC/ANTENNAS.TXT

Comet W100RX telescopic RX whip, segment lengths (mm) from the Comet
W100RX instruction sheet: 200/300/400/510/615/800/986.

firmware/application/external/antenna_length/ui_whipcalc.cpp

@@ -73,6 +73,26 @@ void WhipCalcView::update_result() {
     uint8_t ant_count = 9;                      // Shown antennas counter
     length *= 1000;                             // Get length in mm needed to extend the antenna
     for (antenna_entry antenna : antenna_db) {  // go thru all antennas available
+        if (antenna.elements.empty())
+            continue;
+
+        // Needed quarter-wave length is shorter than the fully-collapsed antenna:
+        // above its fundamental range. Report it as retracted (shortest setting);
+        // a wideband RX whip still receives here via harmonics, but this
+        // quarter-wave model cannot compute those positions. This also avoids an
+        // out-of-bounds read of elements[-1] in the interpolation below.
+        if (length < antenna.elements.front()) {
+            console.write(antenna.label + ": retracted\n");
+            continue;
+        }
+
+        // Needed length is longer than the fully-extended antenna: already maxed
+        // out, so the frequency is below this antenna's range.
+        if (length > antenna.elements.back()) {
+            console.write(antenna.label + ": fully ext.\n");
+            continue;
+        }
+
         uint16_t element, refined_quarter = 0;
         for (element = 0; element < antenna.elements.size(); element++) {
             if (length == antenna.elements[element])  // Exact element in length

sdcard/WHIPCALC/ANTENNAS.TXT

@@ -1,4 +1,5 @@
 #antenna label,elements length in mm, separated by a commas
 ANT700,95,134,175,206,230,245
 ANT500,185,315,450,586,724,862
-Cheap 11cm,118,183,253,326,399,476
+Cheap 11cm,118,183,253,326,399,476
+Comet W100RX,200,300,400,510,615,800,986