Avoid useless warning from Qt

When cloning InsetExternal, the tempname_ member of InsetExternalParams
has yet to be initialized. So, trying to remove it causes the warning
"QFile::remove: Empty or null file name".

(cherry picked from commit 25e6b5da39)

src/insets/InsetExternal.cpp

@@ -98,7 +98,8 @@ TempName::~TempName()
 TempName & TempName::operator=(TempName const & other)
 {
 	if (this != &other) {
-		tempname_.removeFile();
+		if (!tempname_.empty())
+			tempname_.removeFile();
 		support::TempFile f("lyxextXXXXXX.tmp");
 		f.setAutoRemove(false);
 		tempname_ = f.name(); 

status.22x

@@ -176,6 +176,9 @@ What's new
 
 * INTERNALS
 
+- Squash annoying warning from Qt when attempting to delete a non-existent
+  file.
+
 
 * DOCUMENTATION AND LOCALIZATION