Bug fixes for adding extra fields to existing ones.

lib/zip_extra_field.c

diff --git a/lib/zip_extra_field.c b/lib/zip_extra_field.c
index d29cd76..360193b 100644
--- a/lib/zip_extra_field.c
+++ b/lib/zip_extra_field.c
@@ -60,6 +60,8 @@
         if (prev)
             prev->next = def;
         prev = def;
+
+	ef = ef->next;
     }
     
     return head;