For a CPDF_Color object, when set a new pattern to it, disconnect its link with the old pattern

This patch is related to https://pdfium.googlesource.com/pdfium/+/1b9c5c4dc41956b8c5ab17b9a882adf8a2513768

BUG=402260
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/460383004