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/shared/clock-util.c

@@ -55,7 +55,7 @@ int clock_set_hwclock(const struct tm *tm) {
 }
 
 int clock_is_localtime(const char* adjtime_path) {
-        _cleanup_fclose_ FILE *f;
+        _cleanup_fclose_ FILE *f = NULL;
         int r;
 
         if (!adjtime_path)