Fix assertion failure when decoding malform G4 fax image

The position indexes of color elements must be monotonic increasing.
Bail out if the decoded index is less or equal to the previous index.

BUG=pdfium:615

Review-Url: https://codereview.chromium.org/2398033002

core/fxcodec/codec/fx_codec_fax.cpp

diff --git a/core/fxcodec/codec/fx_codec_fax.cpp b/core/fxcodec/codec/fx_codec_fax.cpp
index 336c35b..c121478 100644
--- a/core/fxcodec/codec/fx_codec_fax.cpp
+++ b/core/fxcodec/codec/fx_codec_fax.cpp
@@ -390,6 +390,10 @@
     if (a1 >= columns)
       return TRUE;
 
+    // The position of picture element must be monotonic increasing.
+    if (a0 >= a1)
+      return FALSE;
+
     a0 = a1;
     a0color = !a0color;
   }