Handle empty objects in IsSameTextObject() and free from warning

When both text objects have no items, directly return TRUE;
The last if statement moved inside loop to make the intent more
obvious and free from warning (msvc warns using potentially
uninitialized var itemPer)

R=jun_fang@foxitsoftware.com

Review URL: https://codereview.chromium.org/1815453002 .
diff --git a/core/fpdftext/fpdf_text_int.cpp b/core/fpdftext/fpdf_text_int.cpp
index 9a2a8c1..b6843ba 100644
--- a/core/fpdftext/fpdf_text_int.cpp
+++ b/core/fpdftext/fpdf_text_int.cpp
@@ -1804,7 +1804,12 @@
   if (nPreCount != nCurCount) {
     return FALSE;
   }
-  CPDF_TextObjectItem itemPer, itemCur;
+  // If both objects have no items, consider them same.
+  if (!nPreCount)
+    return TRUE;
+
+  CPDF_TextObjectItem itemPer = {0, 0.0f, 0.0f};
+  CPDF_TextObjectItem itemCur = {0, 0.0f, 0.0f};
   for (int i = 0; i < nPreCount; i++) {
     pTextObj2->GetItemInfo(i, &itemPer);
     pTextObj1->GetItemInfo(i, &itemCur);
@@ -1823,6 +1828,7 @@
   }
   return TRUE;
 }
+
 FX_BOOL CPDF_TextPage::IsSameAsPreTextObject(
     CPDF_TextObject* pTextObj,
     const CPDF_PageObjectList* pObjList,