Fix a segfault in Swatches – issue #3221
As reported in the linked issue, when the user closes any Swatches panel and then right-clicks on another Swatches panel (no matter if it's in the same document), Inkscape segfaults.
This MR fixes the crash by preventing the storage of a stale pointer to a destroyed Swatches panel in the std::map
which keeps track of which panel belongs to which document.
Fixes #3221 (closed)