Add missing changelog item

CHANGELOG-EE

 v 7.13 (Unrelease)
   - Fix git hook validation on initial push to master branch.
   - Reset approvals on push
+  - Fix 500 error when the source project of an MR is deleted 
 
 v 7.12.2
   - Fixed the alignment of project settings icons
@@ -171,4 +172,4 @@ v 6.2.0
   - Use omniauth-ldap nickname attribute as GitLab username
   - Improve group sharing UI for installation with many groups
   - Fix empty LDAP group raises exception
-  - Respect LDAP user filter for git access
+  - Respect LDAP user filter for git access
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