Attempt to improve the EI detection heuristics, for inline images, in streams containing NUL bytes (issue 8823)

Since this patch will now treat (some) `NUL` bytes as "ASCII", the number of `followingBytes` checked are thus increased to (hopefully) reduce the risk of introducing new false positives.

Fixes 8823.

test/pdfs/.gitignore

@@ -58,6 +58,7 @@
 !issue8697.pdf
 !issue8707.pdf
 !issue8798r.pdf
+!issue8823.pdf
 !bad-PageLabels.pdf
 !filled-background.pdf
 !ArabicCIDTrueType.pdf

test/test_manifest.json

@@ -2976,9 +2976,15 @@
     {  "id": "issue8798",
        "file": "pdfs/issue8798r.pdf",
        "md5": "3a0e29f013d9edcceb5d852e37738a77",
+       "link": false,
+       "rounds": 1,
+       "type": "eq"
+    },
+    {  "id": "issue8823",
+       "file": "pdfs/issue8823.pdf",
+       "md5": "ad02d4aa374b315bf1766038d002d57a",
+       "link": false,
        "rounds": 1,
-       "lastPage": 1,
-       "link": true,
        "type": "eq"
     },
     {  "id": "issue8613",