Fix resource leak.

Found by Coverity (CID 1349689).
diff --git a/lib/zip_extra_field.c b/lib/zip_extra_field.c
index c3d7387..258a804 100644
--- a/lib/zip_extra_field.c
+++ b/lib/zip_extra_field.c
@@ -267,6 +267,9 @@
     if (ef_head_p) {
 	*ef_head_p = ef_head;
     }
+    else {
+        _zip_ef_free(ef_head);
+    }
     
     return true;
 }