NULLing pointers after delete

src/atom.cpp

@@ -390,6 +390,8 @@ void Atom::create_avec(const char *style, int narg, char **arg, int trysuffix)
 {
   delete [] atom_style;
   if (avec) delete avec;
+  atom_style = NULL;
+  avec = NULL;
 
   // unset atom style and array existence flags
   // may have been set by old avec

src/force.cpp

@@ -195,6 +195,8 @@ void Force::create_pair(const char *style, int trysuffix)
 {
   delete [] pair_style;
   if (pair) delete pair;
+  pair_style = NULL;
+  pair = NULL;
 
   int sflag;
   pair = new_pair(style,trysuffix,sflag);