Add some casts for gcc 4.8.3 with more-than-default warnings enabled.
diff --git a/lib/zip_extra_field.c b/lib/zip_extra_field.c
index 85cab50..74cf2be 100644
--- a/lib/zip_extra_field.c
+++ b/lib/zip_extra_field.c
@@ -287,7 +287,7 @@
     size = 0;
     for (; ef; ef=ef->next) {
 	if (ef->flags & flags & ZIP_EF_BOTH)
-	    size += 4+ef->size;
+	    size = (zip_uint16_t)(size+4+ef->size);
     }
 
     return size;