Correctly detect the presence of the Adobe specific APP14 JPEG marker (bug 1140761)

According to the specification, http://partners.adobe.com/public/developer/en/ps/sdk/5116.DCT_Filter.pdf#G3.851943, the content of the marker segment should begin with `Adobe`, and not `Adobe\x00` as the code currently look for.

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1140761.
Fixes the colour conversion part of issues 4090 and 5623.

test/pdfs/bug1140761.pdf.link

@@ -0,0 +1 @@
+https://bugzilla.mozilla.org/attachment.cgi?id=8574416

test/test_manifest.json

@@ -1608,6 +1608,15 @@
       "link": true,
       "type": "eq"
     },
+    {  "id": "bug1140761",
+       "file": "pdfs/bug1140761.pdf",
+       "md5": "b74eced7634d4f248dc6265f8225d432",
+       "rounds": 1,
+       "link": true,
+       "firstPage": 1,
+       "lastPage": 1,
+       "type": "eq"
+    },
     {  "id": "bug1011159",
       "file": "pdfs/bug1011159.pdf",
       "md5": "4532e22deb92d4cd2992d0cfe255582a",