no crash after 'unset undefined'

ChangeLog

@@ -1,3 +1,7 @@
+2010-03-20 Holger Vogt
+ * variable.c:450 prevent ngspice from crashing after command 'unset zzz', 
+   when zzz is not defined
+
 2010-03-20 Holger Vogt
  * com_compose.c: comment corrected
  * subckt.c, cktdefs.h, acan.c, ifeval.c, inp2b.c, inpptree.c, cktinit.c:

src/frontend/variable.c

@@ -447,7 +447,7 @@ cp_remvar(char *varname)
         /* Gotta make up a var struct for cp_usrset()... */
         v = alloc(struct variable);
 	ZERO(v, struct variable);
-        v->va_name = varname;
+        v->va_name = copy(varname);
         v->va_type = VT_NUM;
         v->va_bool = 0;
         found = FALSE;