Avoid accidentally getting the entire font file in readNameTable (issue 7020)

In the PDF file in question, some of the 'name' table entries have `record.length === 0`. This becomes problematic in the non-unicode case, since `font.getBytes(0)` will fetch the *entire* stream.
Given that OTS rejects 'name' entries larger than `2^16`, this thus explain the sanitizer errors.

Fixes 7020.

test/pdfs/.gitignore

@@ -19,6 +19,7 @@
 !issue5874.pdf
 !issue6782.pdf
 !issue6961.pdf
+!issue7020.pdf
 !filled-background.pdf
 !ArabicCIDTrueType.pdf
 !ThuluthFeatures.pdf

test/test_manifest.json

@@ -1217,6 +1217,13 @@
        "lastPage": 1,
        "type": "load"
     },
+    {  "id": "issue7020",
+       "file": "pdfs/issue7020.pdf",
+       "md5": "93b464e21c649e64ae92eeafe99fc31b",
+       "link": false,
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "pr4606",
        "file": "pdfs/pr4606.pdf",
        "md5": "6574fde2314648600056bd0e229df98c",