Introduce (Inset|InsetList|Paragraph)::resetBuffer

This avoids an ugly cast like
     setBuffer(*static_cast<Buffer *>(0));
that clang does not appreciate.

src/CutAndPaste.cpp

@@ -652,7 +652,7 @@ void copySelectionHelper(Buffer const & buf, Text const & text,
 	// are not linked to something else.
 	it = copy_pars.begin();
 	for (; it != it_end; ++it) {
-		it->setBuffer(*static_cast<Buffer *>(0));
+		it->resetBuffer();
 		it->setInsetOwner(0);
 	}
 

src/InsetList.cpp

@@ -10,18 +10,15 @@
  */
 
 #include <config.h>
-#include <algorithm>
 
 #include "InsetList.h"
 
-#include "Buffer.h"
-#include "BufferParams.h"
-#include "BranchList.h"
-
-#include "insets/InsetBranch.h"
+#include "insets/Inset.h"
 
 #include "support/debug.h"
 
+#include <algorithm>
+
 using namespace std;
 
 namespace lyx {
@@ -84,6 +81,15 @@ void InsetList::setBuffer(Buffer & b)
 }
 
 
+void InsetList::resetBuffer()
+{
+	List::iterator it = list_.begin();
+	List::iterator end = list_.end();
+	for (; it != end; ++it)
+		it->inset->resetBuffer();
+}
+
+
 InsetList::iterator InsetList::insetIterator(pos_type pos)
 {
 	InsetTable search_elem(pos, 0);

src/InsetList.h

@@ -36,6 +36,8 @@ public:
 	InsetList(InsetList const &, pos_type beg, pos_type end);
 	///
 	void setBuffer(Buffer &);
+	///
+	void resetBuffer();
 
 	///
 	class InsetTable {

src/Paragraph.cpp

@@ -3534,6 +3534,12 @@ void Paragraph::setBuffer(Buffer & b)
 }
 
 
+void Paragraph::resetBuffer()
+{
+	d->insetlist_.resetBuffer();
+}
+
+
 Inset * Paragraph::releaseInset(pos_type pos)
 {
 	Inset * inset = d->insetlist_.release(pos);

src/Paragraph.h

@@ -396,6 +396,8 @@ public:
 	InsetList const & insetList() const;
 	///
 	void setBuffer(Buffer &);
+	///
+	void resetBuffer();
 
 	///
 	bool isHfill(pos_type pos) const;

src/insets/Inset.cpp

@@ -186,6 +186,12 @@ void Inset::setBuffer(Buffer & buffer)
 }
 
 
+void Inset::resetBuffer()
+{
+	buffer_ = nullptr;
+}
+
+
 Buffer & Inset::buffer()
 {
 	if (!buffer_) {

src/insets/Inset.h

@@ -108,6 +108,8 @@ public:
 
 	/// change associated Buffer
 	virtual void setBuffer(Buffer & buffer);
+	/// reset associated Buffer to null value
+	virtual void resetBuffer();
 	/// retrieve associated Buffer
 	virtual Buffer & buffer();
 	virtual Buffer const & buffer() const;