Simplify the handling of unsupported/incorrect markers in src/core/jpg.js

- Re-factor the "incorrect encoding" check, since this can be easily achieved using the general `findNextFileMarker` helper function (with a suitable `startPos` argument).

 - Tweak a condition, to make it easier to see that the end of the data has been reached.

 - Add a reference test for issue 1877, since it's what prompted the "incorrect encoding" check.

src/core/jpg.js

@@ -1031,17 +1031,14 @@ var JpegImage = (function JpegImageClosure() {
             break;
 
           default:
-            if (
-              data[offset - 3] === 0xff &&
-              data[offset - 2] >= 0xc0 &&
-              data[offset - 2] <= 0xfe
-            ) {
-              // could be incorrect encoding -- last 0xFF byte of the previous
-              // block was eaten by the encoder
-              offset -= 3;
-              break;
-            }
-            const nextFileMarker = findNextFileMarker(data, offset - 2);
+            // Could be incorrect encoding -- the last 0xFF byte of the previous
+            // block could have been eaten by the encoder, hence we fallback to
+            // `startPos = offset - 3` when looking for the next valid marker.
+            const nextFileMarker = findNextFileMarker(
+              data,
+              /* currentPos = */ offset - 2,
+              /* startPos = */ offset - 3
+            );
             if (nextFileMarker && nextFileMarker.invalid) {
               warn(
                 "JpegImage.parse - unexpected data, current marker is: " +
@@ -1050,7 +1047,7 @@ var JpegImage = (function JpegImageClosure() {
               offset = nextFileMarker.offset;
               break;
             }
-            if (offset > data.length - 2) {
+            if (offset >= data.length - 1) {
               warn(
                 "JpegImage.parse - reached the end of the image data " +
                   "without finding an EOI marker (0xFFD9)."

test/pdfs/issue1877.pdf.link

@@ -0,0 +1 @@
+https://web.archive.org/web/20160313045333/http://brianmckenna.org/files/IC-Roy.pdf

test/test_manifest.json

@@ -2927,6 +2927,14 @@
       "link": true,
       "type": "eq"
     },
+    {  "id": "issue1877",
+       "file": "pdfs/issue1877.pdf",
+       "md5": "feac01f414f2e6792e4d3174944622f5",
+       "rounds": 1,
+       "link": true,
+       "lastPage": 1,
+       "type": "eq"
+    },
     {  "id": "issue7308",
        "file": "pdfs/issue7308.pdf",
        "md5": "ba2e23d3af93ac2c634d77ccbe2e79d5",