Use the correct stream position when reading maxSizeOfInstructions from the maxp table (issue 9458)

Please refer to the `maxp` table specification, found at https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6maxp.html.

Fixes 9458.

src/core/fonts.js

@@ -2351,7 +2351,7 @@ var Font = (function FontClosure() {
         }
         font.pos += 4;
         maxFunctionDefs = font.getUint16();
-        font.pos += 6;
+        font.pos += 4;
         maxSizeOfInstructions = font.getUint16();
       }
 

test/pdfs/.gitignore

@@ -67,6 +67,7 @@
 !issue9105_reduced.pdf
 !issue9262_reduced.pdf
 !issue9291.pdf
+!issue9458.pdf
 !bad-PageLabels.pdf
 !decodeACSuccessive.pdf
 !filled-background.pdf

test/test_manifest.json

@@ -712,6 +712,13 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "issue9458",
+       "file": "pdfs/issue9458.pdf",
+       "md5": "ee54358d8b2fdc75dc8da5220cf8e8da",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
     {  "id": "issue5501",
        "file": "pdfs/issue5501.pdf",
        "md5": "55a60699728fc92f491a2d7d490474e4",