tree-wide: avoid uninitialized warning on _cleanup_ variables

With some versions of the compiler, the _cleanup_ attr makes it think
the variable might be freed/closed when uninitialized, even though it
cannot happen. The added cost is small enough to be worth the benefit,
and optimized builds will help reduce it even further.

src/cryptsetup/cryptsetup-keyfile.c

@@ -33,7 +33,7 @@ int find_key_file(
         }
 
         STRV_FOREACH(i, search_path) {
-                _cleanup_free_ char *joined;
+                _cleanup_free_ char *joined = NULL;
 
                 joined = path_join(*i, key_file);
                 if (!joined)