BUG: Unterminated object when using PdfWriter with incremental=True (#3345)

Closes #3118.

---------

Co-authored-by: m32 <grzegorz.makarewicz@gmail.com>

Author: m32

pypdf/_writer.py

@@ -1495,7 +1495,7 @@ def _write_increment(self, stream: StreamType) -> None:
             )
         )
         xr.write_to_stream(stream)
-        stream.write(f"\nstartxref\n{xref_location}\n%%EOF\n".encode())  # eof
+        stream.write(f"\nendobj\nstartxref\n{xref_location}\n%%EOF\n".encode())  # eof
 
     def _write_pdf_structure(self, stream: StreamType) -> Tuple[List[int], List[int]]:
         object_positions = []

tests/test_writer.py

@@ -2769,3 +2769,16 @@ def test_insert_filtered_annotations__annotations_are_no_list(caplog):
             "'/Normal'} of type DictionaryObject."
         )
     ]
+
+
+def test_unterminated_object__with_incremental_writer():
+    """Test for #3118"""
+    reader = PdfReader(RESOURCE_ROOT / "bytes.pdf")
+    writer = PdfWriter(reader, incremental=True)
+
+    writer.add_blank_page(72, 72)
+
+    fi = BytesIO()
+    writer.write(fi)
+    b = fi.getvalue()
+    assert b[-39:] == b"\nendstream\nendobj\nstartxref\n1240\n%%EOF\n"