Plug a mem leak

Plug a mem leak

==31063== 98 (24 direct, 74 indirect) bytes in 1 blocks are definitely lost in loss record 946 of 1,136
==31063==    at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==31063==    by 0x4057094: g_malloc (gmem.c:134)
==31063==    by 0x406F2D6: g_slice_alloc (gslice.c:836)
==31063==    by 0x4092383: g_variant_get_child_value (gvariant-core.c:847)
==31063==    by 0x408BE9E: g_variant_get_variant (gvariant.c:709)
==31063==    by 0x40903F5: g_variant_valist_get_nnp (gvariant.c:3767)
==31063==    by 0x40907A9: g_variant_valist_get_leaf (gvariant.c:3884)
==31063==    by 0x4090D10: g_variant_valist_get (gvariant.c:4065)
==31063==    by 0x4090E59: g_variant_valist_get (gvariant.c:4100)
==31063==    by 0x40911B6: g_variant_get_va (gvariant.c:4296)
==31063==    by 0x40910BC: g_variant_get (gvariant.c:4248)
==31063==    by 0x4208DAF: invoke_set_property_in_idle_cb (gdbusconnection.c:3676)

Bug #628346.