util-lib: accept invoking chase_symlinks() with a NULL return parameter

Let's permit invoking chase_symlinks() with a NULL return parameter. If so, the
resolved name is not returned, and call is useful for checking for existance of
a file, without actually returning its ultimate path.

src/basic/fs-util.c

@@ -799,8 +799,10 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags,
                         return -ENOMEM;
         }
 
-        *ret = done;
-        done = NULL;
+        if (ret) {
+                *ret = done;
+                done = NULL;
+        }
 
         return exists;
 }