Prevent boot loop if last open epub crashes on load (#87)

## Summary

* Unset openEpubPath on boot and set once epub fully loaded

## Additional Context

* If an epub was crashing when loading, it was possible to get the
device stuck into a loop. There was no way to get back to the home
screen as we'd always load you back into old epub
* Break this loop by clearing the stored value when we boot, still
jumping to the last open epub, but only resetting that value once the
epub has been fully loaded

src/main.cpp

@@ -194,7 +194,11 @@ void setup() {
   if (APP_STATE.openEpubPath.empty()) {
     onGoHome();
   } else {
-    onGoToReader(APP_STATE.openEpubPath);
+    // Clear app state to avoid getting into a boot loop if the epub doesn't load
+    const auto path = APP_STATE.openEpubPath;
+    APP_STATE.openEpubPath = "";
+    APP_STATE.saveToFile();
+    onGoToReader(path);
   }
 
   // Ensure we're not still holding the power button before leaving setup