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
diff --git a/core/src/fpdfapi/fpdf_page/fpdf_page_colors.cpp b/core/src/fpdfapi/fpdf_page/fpdf_page_colors.cpp
index 8cd26fe..6c2701f 100644
--- a/core/src/fpdfapi/fpdf_page/fpdf_page_colors.cpp
+++ b/core/src/fpdfapi/fpdf_page/fpdf_page_colors.cpp
@@ -1326,6 +1326,7 @@
     PatternValue* pvalue = (PatternValue*)m_pBuffer;
     if (pvalue->m_pPattern && pvalue->m_pPattern->m_pDocument) {
         pDocPageData = pvalue->m_pPattern->m_pDocument->GetPageData();
+        pvalue->m_pPattern->SaveColor(NULL);
         pDocPageData->ReleasePattern(pvalue->m_pPattern->m_pPatternObj);
     }
     pvalue->m_nComps = ncomps;