fix infinite loops when rebuild the cross reference table

BUG=chromium:401189

Review URL: https://codereview.chromium.org/1871373002

core/fpdfapi/fpdf_parser/cpdf_parser.cpp

diff --git a/core/fpdfapi/fpdf_parser/cpdf_parser.cpp b/core/fpdfapi/fpdf_parser/cpdf_parser.cpp
index 97b03f0..f1816f2 100644
--- a/core/fpdfapi/fpdf_parser/cpdf_parser.cpp
+++ b/core/fpdfapi/fpdf_parser/cpdf_parser.cpp
@@ -931,9 +931,9 @@
     }
     pos += size;
 
-    // If the position has not changed at all in a loop iteration, then break
-    // out to prevent infinite looping.
-    if (pos == saved_pos)
+    // If the position has not changed at all or went backwards in a loop
+    // iteration, then break out to prevent infinite looping.
+    if (pos <= saved_pos)
       break;
   }