protocol: fix a memory leak

Discovered by chance with valgrind:

==9036== 11,462 (1,032 direct, 10,430 indirect) bytes in 43 blocks are definitely lost in loss record 1,270 of 1,271 ==9036== at 0x483577F: malloc (vg_replace_malloc.c:299) ==9036== by 0x58FE8D0: g_malloc (gmem.c:99) ==9036== by 0x5916832: g_slice_alloc (gslice.c:1024) ==9036== by 0x5855CD1: g_value_array_new (gvaluearray.c:121) ==9036== by 0x5B249E1: tp_value_array_build (util.c:1115) ==9036== by 0x5B0CFCD: _tp_protocol_parse_manager_file (protocol.c:1869) ==9036== by 0x5AC1578: tp_connection_manager_read_file (connection-manager.c:1080) ==9036== by 0x5AC1578: tp_connection_manager_idle_read_manager_file (connection-manager.c:1131) ==9036== by 0x58F8DD7: g_main_dispatch (gmain.c:3182) ==9036== by 0x58F8DD7: g_main_context_dispatch (gmain.c:3847) ==9036== by 0x58F91C7: g_main_context_iterate.isra.26 (gmain.c:3920) ==9036== by 0x58F94C1: g_main_loop_run (gmain.c:4116) ==9036== by 0x12E1C6: get_vcard_fields (osso-abook-account-manager.c:917) ==9036== by 0x12E9ED: osso_abook_account_manager_init (osso-abook-account-manager.c:1150)