core: fix memory leak in transient units

core: fix memory leak in transient units

Fixes:
==1== HEAP SUMMARY:
==1==     in use at exit: 67,182 bytes in 91 blocks
==1==   total heap usage: 70,485 allocs, 70,394 frees, 42,184,635 bytes
allocated
==1==
==1== 5,742 (696 direct, 5,046 indirect) bytes in 29 blocks are
definitely lost in loss record 4 of 7
==1==    at 0x4C2DD9F: realloc (in
/usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1==    by 0x21ADDD: realloc_multiply (alloc-util.h:67)
==1==    by 0x21BFB0: strv_push (strv.c:448)
==1==    by 0x21C245: strv_consume (strv.c:520)
==1==    by 0x21C33C: strv_extend (strv.c:559)
==1==    by 0x278AD7: unit_write_drop_in (unit.c:3352)
==1==    by 0x278EEB: unit_write_drop_in_private (unit.c:3403)
==1==    by 0x190C21: bus_service_set_transient_property
(dbus-service.c:254)
==1==    by 0x190DBC: bus_service_set_property (dbus-service.c:284)
==1==    by 0x18F00E: bus_unit_set_properties (dbus-unit.c:1226)
==1==    by 0x186F6A: transient_unit_from_message (dbus-manager.c:683)
==1==    by 0x1872B7: method_start_transient_unit (dbus-manager.c:763)
==1==
==1== LEAK SUMMARY:
==1==    definitely lost: 696 bytes in 29 blocks
==1==    indirectly lost: 5,046 bytes in 58 blocks
==1==      possibly lost: 0 bytes in 0 blocks
==1==    still reachable: 61,440 bytes in 4 blocks
==1==         suppressed: 0 bytes in 0 blocks