fix(term): copy_selection() when nothing selected

Fixes the copy_selection() method so that it returns and empty string
when called and nothing is selected. Previously printed an error.

addons/godot_xterm/native/src/terminal.cpp

@@ -648,9 +648,11 @@ String Terminal::_copy_screen(ScreenCopyFunction func) {
 	char *out;
 	PackedByteArray data;
 
-	data.resize(func(screen, &out));
-	memcpy(data.ptrw(), out, data.size());
-	std::free(out);
+	data.resize(std::max(func(screen, &out), 0));
+	if (data.size() > 0) {
+		memcpy(data.ptrw(), out, data.size());
+		std::free(out);
+	}
 
 	return data.get_string_from_utf8();
 }

test/test_terminal.gd

@@ -222,6 +222,9 @@ class TestCopy:
 		subject.write(text)
 		assert_string_contains(subject.copy_all(), text)
 
+	func test_copy_selection_when_nothing_selected():
+		assert_eq(subject.copy_selection(), "")
+
 
 class TestClear:
 	extends TerminalTest