plug memory leak detected by coverity scan

src/EXTRA-DUMP/dump_extxyz.cpp

@@ -63,6 +63,13 @@ DumpExtXYZ::DumpExtXYZ(LAMMPS *lmp, int narg, char **arg) :
 
 /* ---------------------------------------------------------------------- */
 
+DumpExtXYZ::~DumpExtXYZ()
+{
+  delete[] properties_string;
+}
+
+/* ---------------------------------------------------------------------- */
+
 void DumpExtXYZ::update_properties()
 {
   // How many per-atom elements we buffer

src/EXTRA-DUMP/dump_extxyz.h

@@ -26,6 +26,7 @@ namespace LAMMPS_NS {
 class DumpExtXYZ : public DumpXYZ {
  public:
   DumpExtXYZ(class LAMMPS *, int, char **);
+  ~DumpExtXYZ() override;
 
  protected:
   int with_vel;