Fix error reading concatenated pdfs

src/core/obj.js

@@ -1051,12 +1051,13 @@ var XRef = (function XRefClosure() {
           trailers.push(position);
           position += skipUntil(buffer, position, startxrefBytes);
         } else if ((m = /^(\d+)\s+(\d+)\s+obj\b/.exec(token))) {
-          this.entries[m[1]] = {
-            offset: position,
-            gen: m[2] | 0,
-            uncompressed: true
-          };
-
+          if (typeof this.entries[m[1]] === 'undefined') {
+            this.entries[m[1]] = {
+              offset: position,
+              gen: m[2] | 0,
+              uncompressed: true
+            };
+          }
           var contentLength = skipUntil(buffer, position, endobjBytes) + 7;
           var content = buffer.subarray(position, position + contentLength);
 

test/pdfs/.gitignore

@@ -123,3 +123,4 @@
 !issue5481.pdf
 !issue5567.pdf
 !issue5701.pdf
+!issue5909.pdf

test/test_manifest.json

@@ -2087,5 +2087,11 @@
        "md5": "d5b37f8bf1b3aafa1b4fcf19ebdc7c74",
        "rounds": 1,
        "type": "eq"
+    },
+    {  "id": "issue5909",
+       "file": "pdfs/issue5909.pdf",
+       "md5": "51a724136c0c10008bd061a78ea4b8fc",
+       "rounds": 1,
+       "type": "load"
     }
 ]