Let Lexer.getObj return a dummy-Cmd for commands that start with a non-visible ASCII character (issue 13999)

This way we avoid breaking badly generated PDF documents where a non-visible ASCII character is "glued" to a valid command.

test/pdfs/issue13999.pdf.link

@@ -0,0 +1 @@
+https://github.com/mozilla/pdf.js/files/7147166/default.pdf

test/test_manifest.json

@@ -5885,5 +5885,13 @@
        "rounds": 1,
        "enableXfa": true,
        "type": "eq"
+    },
+    {  "id": "issue13999",
+       "file": "pdfs/issue13999.pdf",
+       "md5": "8829c76887e7827390e9a5f73cab23f2",
+       "link": true,
+       "rounds": 1,
+       "lastPage": 1,
+       "type": "eq"
     }
 ]

test/unit/parser_spec.js

@@ -13,9 +13,9 @@
  * limitations under the License.
  */
 
+import { Cmd, EOF, Name } from "../../src/core/primitives.js";
 import { Lexer, Linearization, Parser } from "../../src/core/parser.js";
 import { FormatError } from "../../src/shared/util.js";
-import { Name } from "../../src/core/primitives.js";
 import { StringStream } from "../../src/core/stream.js";
 
 describe("parser", function () {
@@ -217,6 +217,32 @@ describe("parser", function () {
         }
       });
     });
+
+    describe("getObj", function () {
+      it(
+        "should stop immediately when the start of a command is " +
+          "a non-visible ASCII character (issue 13999)",
+        function () {
+          const input = new StringStream("\x14q\nQ");
+          const lexer = new Lexer(input);
+
+          let obj = lexer.getObj();
+          expect(obj instanceof Cmd).toEqual(true);
+          expect(obj.cmd).toEqual("\x14");
+
+          obj = lexer.getObj();
+          expect(obj instanceof Cmd).toEqual(true);
+          expect(obj.cmd).toEqual("q");
+
+          obj = lexer.getObj();
+          expect(obj instanceof Cmd).toEqual(true);
+          expect(obj.cmd).toEqual("Q");
+
+          obj = lexer.getObj();
+          expect(obj).toEqual(EOF);
+        }
+      );
+    });
   });
 
   describe("Linearization", function () {