Reject out-of-range RPC values instead of narrowing them silently.

Guards choke group max, set-port and info-hash length; parsing now uses std::from_chars.

src/command_helpers.h

@@ -1,12 +1,25 @@
 #ifndef RTORRENT_UTILS_COMMAND_HELPERS_H
 #define RTORRENT_UTILS_COMMAND_HELPERS_H
 
+#include <cstdint>
+#include <limits>
+#include <string>
+#include <torrent/exceptions.h>
+
 #include "rpc/command.h"
 #include "rpc/parse_commands.h"
 #include "rpc/object_storage.h"
 
 void initialize_commands();
 
+inline uint16_t
+checked_port_value(int64_t value, const char* label) {
+  if (value < 0 || value > std::numeric_limits<uint16_t>::max())
+    throw torrent::input_error(std::string("Invalid ") + label + " port number.");
+
+  return static_cast<uint16_t>(value);
+}
+
 //
 // Aliases with CMD_* for the below
 //