Add a NOTREACHED() in CFX_StockFontArray::GetFont().

The index passed in an enum of type CFX_FontMapper::StandardFont. It
should never go out of bounds in CFX_StockFontArray.

Change-Id: I04d0ebbabb9fc772e79dbbf9408913445164b781
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/59111
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/core/fpdfapi/font/cfx_stockfontarray.cpp b/core/fpdfapi/font/cfx_stockfontarray.cpp
index 2c3bca4..a8d8597 100644
--- a/core/fpdfapi/font/cfx_stockfontarray.cpp
+++ b/core/fpdfapi/font/cfx_stockfontarray.cpp
@@ -26,9 +26,10 @@
 
 RetainPtr<CPDF_Font> CFX_StockFontArray::GetFont(
     CFX_FontMapper::StandardFont index) const {
-  if (index >= FX_ArraySize(m_StockFonts))
-    return nullptr;
-  return m_StockFonts[index];
+  if (index < FX_ArraySize(m_StockFonts))
+    return m_StockFonts[index];
+  NOTREACHED();
+  return nullptr;
 }
 
 void CFX_StockFontArray::SetFont(CFX_FontMapper::StandardFont index,