core: fix memory leak on set-default, enable, disable etc

core: fix memory leak on set-default, enable, disable etc

Fixes:
==1==    by 0x23E44C: remove_marked_symlinks_fd (install.c:453)
==1==    by 0x23E256: remove_marked_symlinks_fd (install.c:405)
==1==    by 0x23E630: remove_marked_symlinks (install.c:494)
==1==    by 0x2427A0: unit_file_disable (install.c:1876)
==1==    by 0x18A633: method_disable_unit_files_generic (dbus-manager.c:1760)
==1==    by 0x18A6CA: method_disable_unit_files (dbus-manager.c:1768)
==1==    by 0x1D8146: method_callbacks_run (bus-objects.c:420)
==1==    by 0x1DA9D8: object_find_and_run (bus-objects.c:1257)
==1==    by 0x1DB01A: bus_process_object (bus-objects.c:1373)
==1==
==1== 228 (48 direct, 180 indirect) bytes in 2 blocks are definitely lost in loss record 8 of 14
==1==    at 0x4C2BBCF: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1==    by 0x4C2DE2F: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1==    by 0x23DA60: unit_file_changes_add (install.c:233)
==1==    by 0x23DDB2: create_symlink (install.c:298)
==1==    by 0x240C5C: install_info_symlink_wants (install.c:1328)
==1==    by 0x240FC8: install_info_apply (install.c:1384)
==1==    by 0x241211: install_context_apply (install.c:1439)
==1==    by 0x242563: unit_file_enable (install.c:1830)
==1==    by 0x18A06E: method_enable_unit_files_generic (dbus-manager.c:1650)
==1==    by 0x18A141: method_enable_unit_files (dbus-manager.c:1660)
==1==    by 0x1D8146: method_callbacks_run (bus-objects.c:420)
==1==    by 0x1DA9D8: object_find_and_run (bus-objects.c:1257)
==1==
==1== 467 (144 direct, 323 indirect) bytes in 3 blocks are definitely lost in loss record 9 of 14
==1==    at 0x4C2DD9F: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1==    by 0x23DA60: unit_file_changes_add (install.c:233)
==1==    by 0x23DE97: create_symlink (install.c:320)
==1==    by 0x242CFC: unit_file_set_default (install.c:1951)
==1==    by 0x18A881: method_set_default_target (dbus-manager.c:1802)
==1==    by 0x1D8146: method_callbacks_run (bus-objects.c:420)
==1==    by 0x1DA9D8: object_find_and_run (bus-objects.c:1257)
==1==    by 0x1DB01A: bus_process_object (bus-objects.c:1373)
==1==    by 0x259143: process_message (sd-bus.c:2567)
==1==    by 0x259326: process_running (sd-bus.c:2609)
==1==    by 0x259BDC: bus_process_internal (sd-bus.c:2798)
==1==    by 0x259CAD: sd_bus_process (sd-bus.c:2817)
==1==
==1== LEAK SUMMARY:
==1==    definitely lost: 216 bytes in 6 blocks
==1==    indirectly lost: 560 bytes in 14 blocks
==1==      possibly lost: 0 bytes in 0 blocks
==1==    still reachable: 65,536 bytes in 5 blocks
==1==         suppressed: 0 bytes in 0 blocks
==1== Reachable blocks (those to which a pointer was found) are not shown.
==1== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==1==