Fix reading number of HTMX metrics.

The length of the HHEA table can be incorrect, so it is better to
read the number of metrics offset from beginning of table instead.

test/pdfs/.gitignore

@@ -125,6 +125,7 @@
 !issue6541.pdf
 !issue2948.pdf
 !issue6231_1.pdf
+!issue10402.pdf
 !issue7074_reduced.pdf
 !issue6413.pdf
 !issue4630.pdf

test/test_manifest.json

@@ -3429,6 +3429,12 @@
       "type": "eq",
       "about": "True type font with encoding dict with no base encoding but with differences."
     },
+    {  "id": "issue10402",
+       "file": "pdfs/issue10402.pdf",
+       "md5": "7936bd34d7c0aebd0a864b5aa98aa1b4",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "issue3064-text",
        "file": "pdfs/issue3064.pdf",
        "md5": "0307415b7d69b13acaf8bd4285d9544b",