ole32/stg_bigblockfile: Check page before use (Coverity).

dlls/ole32/stg_bigblockfile.c

@@ -861,18 +861,18 @@ static HRESULT WINAPI ImplBIGBLOCKFILE_WriteAt(
         MappedPage *page = BIGBLOCKFILE_GetMappedView(This, page_index);
 
         TRACE("page %i,  offset %u, bytes_to_page %u, bytes_left %u\n",
-            page->page_index, offset_in_page, bytes_to_page, bytes_left);
+            page ? page->page_index : 0, offset_in_page, bytes_to_page, bytes_left);
 
-        if (page->mapped_bytes < bytes_to_page)
+        if (!page)
         {
-            ERR("Not enough bytes mapped to the page. This should never happen\n");
+            ERR("Unable to get a page to write. This should never happen\n");
             rc = E_FAIL;
             break;
         }
 
-        if (!page)
+        if (page->mapped_bytes < bytes_to_page)
         {
-            ERR("Unable to get a page to write. This should never happen\n");
+            ERR("Not enough bytes mapped to the page. This should never happen\n");
             rc = E_FAIL;
             break;
         }