plug memory leaks in couple examples

examples/COUPLE/plugin/simple.c

@@ -148,6 +148,7 @@ int main(int narg, char **arg)
 
     double *fx = (double *) plugin->extract_variable(lmp,"fx",(char *)"all");
     printf("Force on 1 atom via extract_variable: %g\n",fx[0]);
+    plugin->free(fx);
   }
 
   /* use commands_string() and commands_list() to invoke more commands */

examples/COUPLE/simple/simple.c

@@ -121,6 +121,7 @@ int main(int narg, char **arg)
 
     double *fx = (double *) lammps_extract_variable(lmp,"fx","all");
     printf("Force on 1 atom via extract_variable: %g\n",fx[0]);
+    lammps_free(fx);
   }
 
   /* use commands_string() and commands_list() to invoke more commands */

examples/COUPLE/simple/simple.cpp

@@ -131,6 +131,7 @@ int main(int narg, char **arg)
     double *fx = (double *) 
       lammps_extract_variable(lmp,(char *) "fx",(char *) "all");
     printf("Force on 1 atom via extract_variable: %g\n",fx[0]);
+    lammps_free(fx);
   }
 
   // use commands_string() and commands_list() to invoke more commands