fix: load access fault crash (#1370)

## Summary

Fixes a crash (load access fault) when opening EPUB chapters whose first
text block exceeds 750 words.

## Changes

* **Crash fix (`addLineToPage`)**: Added a null guard for `currentPage`.
If `makePages()` hasn't run yet (which can happen when the first block
triggers the "text block too long" split path), the page is now created
on demand.
* **Layout fix (`characterData`)**: The early-split path previously used
`viewportWidth`, ignoring CSS margins and padding. It now computes
`effectiveWidth` using `totalHorizontalInset()`, consistent with
`makePages()`.

## Additional Context

* Crash signature: `MCAUSE=0x5` (load access fault), `A0=0x0` (`Page*`
null), `MTVAL=0x4 / 0x8` (offsets into `Page::elements`).
* Confirmed in two user reports reported in #1328
* Tested on PR #1357 (not on `master`).

---

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lib/Epub/Epub/parsers/ChapterHtmlSlimParser.cpp

@@ -772,8 +772,12 @@ void XMLCALL ChapterHtmlSlimParser::characterData(void* userData, const XML_Char
   // Spotted when reading Intermezzo, there are some really long text blocks in there.
   if (self->currentTextBlock->size() > 750) {
     LOG_DBG("EHP", "Text block too long, splitting into multiple pages");
+    const int horizontalInset = self->currentTextBlock->getBlockStyle().totalHorizontalInset();
+    const uint16_t effectiveWidth = (horizontalInset < self->viewportWidth)
+                                        ? static_cast<uint16_t>(self->viewportWidth - horizontalInset)
+                                        : self->viewportWidth;
     self->currentTextBlock->layoutAndExtractLines(
-        self->renderer, self->fontId, self->viewportWidth,
+        self->renderer, self->fontId, effectiveWidth,
         [self](const std::shared_ptr<TextBlock>& textBlock) { self->addLineToPage(textBlock); }, false);
   }
 }
@@ -1020,6 +1024,11 @@ bool ChapterHtmlSlimParser::parseAndBuildPages() {
 void ChapterHtmlSlimParser::addLineToPage(std::shared_ptr<TextBlock> line) {
   const int lineHeight = renderer.getLineHeight(fontId) * lineCompression;
 
+  if (!currentPage) {
+    currentPage.reset(new Page());
+    currentPageNextY = 0;
+  }
+
   if (currentPageNextY + lineHeight > viewportHeight) {
     completePageFn(std::move(currentPage));
     completedPageCount++;