Cast NULL, for gcc 4.7.2.

lib/zip.h

diff --git a/lib/zip.h b/lib/zip.h
index 67ef791..7b53ff7 100644
--- a/lib/zip.h
+++ b/lib/zip.h
@@ -256,7 +256,7 @@
 };
 
 typedef struct zip_source_args_seek zip_source_args_seek_t;
-#define ZIP_SOURCE_GET_ARGS(type, data, len, error) ((len) < sizeof(type) ? zip_error_set((error), ZIP_ER_INVAL, 0), NULL : (type *)(data))
+#define ZIP_SOURCE_GET_ARGS(type, data, len, error) ((len) < sizeof(type) ? zip_error_set((error), ZIP_ER_INVAL, 0), (type *)NULL : (type *)(data))
 
 
 /* error information */