[pipsolar] Guard handle_qmod_ against empty message (#16085)

esphome/components/pipsolar/pipsolar.cpp

@@ -433,13 +433,17 @@ void Pipsolar::handle_qpigs_(const char *message) {
 }
 
 void Pipsolar::handle_qmod_(const char *message) {
-  std::string mode;
-  char device_mode = char(message[1]);
   if (this->last_qmod_) {
     this->last_qmod_->publish_state(message);
   }
+  // QMOD response is "(M" where M is the device-mode character. Bail out if the
+  // message is shorter than 2 chars (e.g. empty error response from
+  // handle_poll_error_) — reading message[1] would otherwise be out of bounds.
+  if (message[0] == '\0' || message[1] == '\0')
+    return;
   if (this->device_mode_) {
-    mode = device_mode;
+    std::string mode;
+    mode = char(message[1]);
     this->device_mode_->publish_state(mode);
   }
 }