Fix PDFium abuse that leads to a crash on Windows ARM (#3460)

Signed-off-by: Jared Van Bortel <jared@nomic.ai>

gpt4all-chat/src/database.cpp

@@ -1209,7 +1209,6 @@ public:
             FPDF_ClosePage(m_page);
         if (m_doc)
             FPDF_CloseDocument(m_doc);
-        FPDF_DestroyLibrary();
     }
 
     int page() const override { return m_currentPage; }
@@ -1224,7 +1223,7 @@ private:
                     return std::nullopt;
 
                 if (m_page)
-                    FPDF_ClosePage(m_page);
+                    FPDF_ClosePage(std::exchange(m_page, nullptr));
                 m_page = FPDF_LoadPage(m_doc, m_currentPage++);
                 if (!m_page)
                     throw std::runtime_error("Failed to load page.");

gpt4all-chat/src/main.cpp

@@ -181,5 +181,9 @@ int main(int argc, char *argv[])
     // Otherwise, we can get a heap-use-after-free inside of llama.cpp.
     ChatListModel::globalInstance()->destroyChats();
 
+#ifndef GPT4ALL_USE_QTPDF
+    FPDF_DestroyLibrary();
+#endif
+
     return res;
 }