sftp: Fix a memory on error in sftp_opendir().

(cherry picked from commit b5c4b090da)

src/sftp.c

@@ -940,6 +940,7 @@ sftp_dir sftp_opendir(sftp_session sftp, const char *path){
         dir = malloc(sizeof(struct sftp_dir_struct));
         if (dir == NULL) {
           ssh_set_error_oom(sftp->session);
+          free(file);
           return NULL;
         }
         ZERO_STRUCTP(dir);