Prevent readCmapTable from failing if the cmap is missing in TrueType fonts

Fixes http://arrow.dit.ie/cgi/viewcontent.cgi?article=1000&context=aaschadpoth#page=3.

test/pdfs/.gitignore

@@ -2,6 +2,7 @@
 *.error
 
 !tracemonkey.pdf
+!TrueType_without_cmap.pdf
 !franz.pdf
 !franz_2.pdf
 !xref_command_missing.pdf

test/test_manifest.json

@@ -1196,6 +1196,13 @@
        "rounds": 1,
        "type": "eq"
     },
+    {  "id": "TrueType_without_cmap",
+       "file": "pdfs/TrueType_without_cmap.pdf",
+       "md5": "afca8bb11f2e1f7298b4e5dd85785fb0",
+       "link": false,
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "issue3323",
        "file": "pdfs/issue3323.pdf",
        "md5": "1a14ff574013caeafa9d598269988764",