core/unit: fix unit_add_target_dependencies() for units with no dependencies

For units without any dependencies, r needs to be initialized to 0.
Otherwise, the return value of unit_add_target_dependencies() is
unspecified.

src/core/unit.c

@@ -959,7 +959,7 @@ static int unit_add_target_dependencies(Unit *u) {
         Unit *target;
         Iterator i;
         unsigned k;
-        int r;
+        int r = 0;
 
         assert(u);