fix: go to prev page on the first one, get teleported to the end of book (#970)

## Summary

1. Go to the first page in a .epub file.
2. Hit `Up` button
3. Get teleported to the last page :)

`TxtRenderActivity` seems to have this if check, but EPUB one does not.

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src/activities/reader/EpubReaderActivity.cpp

@@ -337,7 +337,7 @@ void EpubReaderActivity::loop() {
   if (prevTriggered) {
     if (section->currentPage > 0) {
       section->currentPage--;
-    } else {
+    } else if (currentSpineIndex > 0) {
       // We don't want to delete the section mid-render, so grab the semaphore
       {
         RenderLock lock(*this);