Ensure that entries in the Differences array of Type1 fonts are either numbers or names

This patch is yet another installment in the (never ending) series of bugs in PDF files with non-embedded fonts.

Fixes http://www.int.washington.edu/talks/WorkShops/int_08_37W/People/Franz_M/Franz.pdf.

test/pdfs/.gitignore

@@ -2,6 +2,7 @@
 *.error
 
 !tracemonkey.pdf
+!franz.pdf
 !issue2391-1.pdf
 !issue2391-2.pdf
 !filled-background.pdf

test/pdfs/franz.pdf

@@ -0,0 +1,81 @@
+%PDF-1.7
+%âãÏÓ
+1 0 obj 128
+endobj 
+2 0 obj 157
+endobj 
+3 0 obj 
+<<
+/Type /Encoding
+/Differences [0 /.notdef 1 /dotaccent /fi /fl /fraction /hungarumlaut /Lslash /lslash /ogonek /ring 10 /.notdef 11 /breve /minus 13 /.notdef 14 /Zcaron /zcaron /caron /dotlessi /dotlessj /ff /ffi /ffl 22 /.notdef 30 /grave /quotesingle /space /exclam /quotedbl /numbersign /dollar /percent /ampersand /quoteright /parenleft /parenright /asterisk /plus /comma /hyphen /period /slash /zero /one /two /three /four /five /six /seven /eight /nine /colon /semicolon /less /equal /greater /question /at /A /B /C /D /E /F /G /H /I /J /K /L /M /N /O /P /Q /R /S /T /U /V /W /X /Y /Z /bracketleft /backslash /bracketright /asciicircum /underscore /quoteleft /a /b /c /d /e /f /g /h /i /j /k /l /m /n /o /p /q /r /s /t /u /v /w /x /y /z /braceleft /bar /braceright /asciitilde 127 /.notdef 1 0 R /Euro 129 /.notdef 130 /quotesinglbase /florin /quotedblbase /ellipsis /dagger /daggerdbl /circumflex /perthousand /Scaron /guilsinglleft /OE 141 /.notdef 147 /quotedblleft /quotedblright /bullet /endash /emdash /tilde /trademark /scaron /guilsinglright /oe 2 0 R /.notdef 159 /Ydieresis 160 /.notdef 161 /exclamdown /cent /sterling /currency /yen /brokenbar /section /dieresis /copyright /ordfeminine /guillemotleft /logicalnot /hyphen /registered /macron /degree /plusminus /twosuperior /threesuperior /acute /mu /paragraph /periodcentered /cedilla /onesuperior /ordmasculine /guillemotright /onequarter /onehalf /threequarters /questiondown /Agrave /Aacute /Acircumflex /Atilde /Adieresis /Aring /AE /Ccedilla /Egrave /Eacute /Ecircumflex /Edieresis /Igrave /Iacute /Icircumflex /Idieresis /Eth /Ntilde /Ograve /Oacute /Ocircumflex /Otilde /Odieresis /multiply /Oslash /Ugrave /Uacute /Ucircumflex /Udieresis /Yacute /Thorn /germandbls /agrave /aacute /acircumflex /atilde /adieresis /aring /ae /ccedilla /egrave /eacute /ecircumflex /edieresis /igrave /iacute /icircumflex /idieresis /eth /ntilde /ograve /oacute /ocircumflex /otilde /odieresis /divide /oslash /ugrave /uacute /ucircumflex /udieresis /yacute /thorn /ydieresis]
+>>
+endobj 
+4 0 obj 
+<<
+/Pages 5 0 R
+/Type /Catalog
+>>
+endobj 
+5 0 obj 
+<<
+/MediaBox [0 0 200 50]
+/Kids [6 0 R]
+/Count 1
+/Type /Pages
+>>
+endobj 
+6 0 obj 
+<<
+/Parent 5 0 R
+/MediaBox [0 0 200 50]
+/Resources 
+<<
+/Font 
+<<
+/F1 7 0 R
+>>
+>>
+/Contents 8 0 R
+/Type /Page
+>>
+endobj 
+7 0 obj 
+<<
+/BaseFont /Helvetica-Bold
+/Subtype /Type1
+/Type /Font
+/Encoding 3 0 R
+>>
+endobj 
+8 0 obj 
+<<
+/Length 42
+>>
+stream
+BT
+10 20 TD
+/F1 20 Tf
+(Hello World) Tj
+ET
+
+endstream 
+endobj xref
+0 9
+0000000000 65535 f 
+0000000015 00000 n 
+0000000035 00000 n 
+0000000055 00000 n 
+0000002111 00000 n 
+0000002162 00000 n 
+0000002244 00000 n 
+0000002373 00000 n 
+0000002466 00000 n 
+trailer
+
+<<
+/Root 4 0 R
+/Size 9
+>>
+startxref
+2560
+%%EOF

test/test_manifest.json

@@ -490,6 +490,14 @@
        "lastPage": 1,
        "type": "eq"
     },
+    {  "id": "franz",
+       "file": "pdfs/franz.pdf",
+       "md5": "12f0bbdec09900cbdd86e1737ed5f992",
+       "link": false,
+       "rounds": 1,
+       "type": "eq",
+       "about": "Type1 font with |Ref|s in the Differences array of the Encoding dictionary."
+    },
     {  "id": "issue4934",
        "file": "pdfs/issue4934.pdf",
        "md5": "6099da44f677702ae65a648b51a2226d",