Handle the Encoding being a dictionary in PartialEvaluator_preEvaluateFont (bug 1157493)

*This is a regression from PR 4423.*

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1157493.

test/pdfs/.gitignore

@@ -32,6 +32,7 @@
 !bug894572.pdf
 !bug911034.pdf
 !bug1108301.pdf
+!bug1157493.pdf
 !pdfjsbad1586.pdf
 !freeculture.pdf
 !pdfkit_compressed.pdf

test/test_manifest.json

@@ -1164,6 +1164,13 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "bug1157493",
+       "file": "pdfs/bug1157493.pdf",
+       "md5": "df96eddacf186c28a91e699800180c4f",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
     {  "id": "issue1466",
       "file": "pdfs/issue1466.pdf",
       "md5": "8a8877432e5bb10cfd50d60488d947bb",