ui: read the signal count under the lock in the badge bar; validate the drop payload

src/ui_signals.cpp

@@ -557,7 +557,11 @@ gboolean on_drop_channel(GtkDropTarget*, const GValue* value, double x, double y
   // Where the drop landed decides the row: one area covers them all.
   const int row = row_at(*app, x, y);
   const char* text = g_value_get_string(value);
-  const std::size_t signal = static_cast<std::size_t>(std::atoll(text ? text : "0"));
+  if (text == nullptr || *text == '\0') return FALSE;
+  char* end = nullptr;
+  const unsigned long long parsed = std::strtoull(text, &end, 10);
+  if (end == nullptr || *end != '\0') return FALSE;
+  const std::size_t signal = static_cast<std::size_t>(parsed);
   if (!assign_signal_to_row(*app, signal, row)) return FALSE;
   cancel_drag_on(*app, row);
   return TRUE;