fix: dont crash on cutting with ghost paste

[un000000]

Fixes #123

This pr fixes crash at the tradeoff of introducing new visual (mostly harmless) bug: When cutting with ghost paste (explained below) hovering on cursor, the ghost paste actually stays, but is invisible. Left clicking will paste that invisible content.
Steps to reproduce this visual bug are same as for crash:

Make an area selection that has some items (preferably about one in-game screen or bigger)
Cut with ctrl + x.
Undo cutting with ctrl + z.
Paste clipboard with ctrl + v (this will cancel the selection). <-- this is the 'ghost paste' i'm referring to
Undo selection cancel with ctrl + z.
Cut with ctrl + x. <-- you still have 'ghost paste' on cursor, but its invisible now

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
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[gemini-code-assist]

Code Review

This pull request updates the CopyBuffer::clear method to safely delete tiles and prevent dangling pointers by clearing g_gui.secondary_map. The reviewer recommended calling g_gui.EndPasting() instead of manually setting the pointer to nullptr to ensure the internal pasting state is correctly reset, which addresses a potential visual bug.

[gemini-code-assist]

Instead of manually setting secondary_map to nullptr, you should call g_gui.EndPasting(). This will not only clear the secondary_map pointer but also reset the pasting flag to false. This fixes the "visual bug" mentioned in the PR description where the ghost paste remains active (but invisible) and allows for accidental pasting after the buffer has been cleared.

	if (g_gui.secondary_map == tiles) {
		g_gui.EndPasting();
	}