core: drop the dependency dropin name from the mask log line

The log line reported the file name of the masked dependency symlink, except
that entry is not assigned and is NULL at that point.

(cherry picked from commit 6e6114dd6cafb8d7c9585cf1d4ee58866db42d99)

src/core/load-dropin.c

@@ -42,8 +42,8 @@ static int process_deps(Unit *u, UnitDependency dependency, const char *dir_suff
 
                 if (null_or_empty_path(*p) > 0) {
                         /* an error usually means an invalid symlink, which is not a mask */
-                        log_unit_debug(u, "%s dependency on %s is masked by %s, ignoring.",
-                                       unit_dependency_to_string(dependency), entry, *p);
+                        log_unit_debug(u, "%s dependency is masked by %s, ignoring.",
+                                       unit_dependency_to_string(dependency), *p);
                         continue;
                 }