After iconv init reset ref normalize error

The iconv init was accidentally clearing the default error state
during reference normalization. This resets so that normalization
errors will be detected correctly.

src/refs.c

@@ -755,6 +755,7 @@ int git_reference__normalize_name(
 		if ((error = git_path_iconv_init_precompose(&ic)) < 0 ||
 			(error = git_path_iconv(&ic, &current, &namelen)) < 0)
 			goto cleanup;
+		error = GIT_EINVALIDSPEC;
 	}
 
 	while (true) {