all: avoid bogus compiler warning about uninitialized variable

With LTO and optimizations, gcc issues several bogus
"maybe-uninitialized" warnings.

Work-around them by initializing the variables.

shared/systemd/src/basic/hashmap.c

@@ -890,7 +890,8 @@ void internal_hashmap_clear(HashmapBase *h, free_func_t default_free_key, free_f
                  * themselves from our hash table a second time, the entry is already gone. */
 
                 while (internal_hashmap_size(h) > 0) {
-                        void *v, *k;
+                        void *k = NULL;
+                        void *v;
 
                         v = internal_hashmap_first_key_and_value(h, true, &k);