tree-wide: Fix false positives on newer gcc

Recent gcc versions have started to trigger false positive
maybe-uninitialized warnings. Let's make sure we initialize
variables annotated with _cleanup_ to avoid these.

src/test/test-string-util.c

@@ -259,7 +259,7 @@ TEST(strextend_with_separator) {
 }
 
 TEST(strrep) {
-        _cleanup_free_ char *one, *three, *zero;
+        _cleanup_free_ char *one = NULL, *three = NULL, *zero = NULL;
         one = strrep("waldo", 1);
         three = strrep("waldo", 3);
         zero = strrep("waldo", 0);