Adjust rig_list_foreach to allow use for unregistering

This patch changes rig_list_foreach such  that the called function may
call rig_unregister without having to access freed memory. This avoids
a valgrind  MemCheck and makes  it possible  to clean up  teh rig_list
database.

src/register.c

@@ -298,10 +298,13 @@ int HAMLIB_API rig_list_foreach(int (*cfunc)(const struct rig_caps*, rig_ptr_t),
     return -RIG_EINVAL;
 
   for (i=0; i<RIGLSTHASHSZ; i++) {
-		for (p=rig_hash_table[i]; p; p=p->next)
+    struct rig_list * next = NULL;
+    for (p=rig_hash_table[i]; p; p=next) {
+      next = p->next;		/* read before call in case it is unregistered */
       if ((*cfunc)(p->caps,data) == 0)
 	return RIG_OK;
     }
+  }
 
   return RIG_OK;
 }