VMOD blob: correct a special case in object finalization.

Don't attempt to free the static empty string that the VMOD assigns
for empty encodings.

lib/libvmod_blob/vmod_blob.c

@@ -306,7 +306,7 @@ vmod_blob__fini(struct vmod_blob_blob **blobp)
 		b->blob.priv = NULL;
 	}
 	for (int i = 0; i < __MAX_ENCODING; i++)
-		if (b->encoding[i] != NULL && b->encoding[i][0] != '\0') {
+		if (b->encoding[i] != NULL && b->encoding[i] != empty) {
 			free(b->encoding[i]);
 			b->encoding[i] = NULL;
 		}