supplicant: fix a memory leak

==30980== 8 bytes in 1 blocks are definitely lost in loss record 1,117 of 6,137
==30980==    at 0x4841C38: malloc (vg_replace_malloc.c:309)
==30980==    by 0x4A246C7: g_malloc (gmem.c:106)
==30980==    by 0x4A4A4BB: g_variant_get_strv (gvariant.c:1607)
==30980==    by 0x4A4CA73: g_variant_valist_get_nnp (gvariant.c:4901)
==30980==    by 0x4A4CA73: g_variant_valist_get_leaf (gvariant.c:5058)
==30980==    by 0x4A4CA73: g_variant_valist_get (gvariant.c:5239)
==30980==    by 0x4A4D11D: g_variant_get_va (gvariant.c:5502)
==30980==    by 0x4A4D1BD: g_variant_lookup (gvariant.c:989)
==30980==    by 0xE9389: parse_capabilities (nm-supplicant-interface.c:1241)
==30980==    by 0xEBF99: _properties_changed_main (nm-supplicant-interface.c:1941)
==30980==    by 0xEF549: _properties_changed (nm-supplicant-interface.c:2867)
==30980==    by 0xEF7ED: _get_all_main_cb (nm-supplicant-interface.c:2972)
==30980==    by 0x262057: _nm_dbus_connection_call_default_cb (nm-dbus-aux.c:70)
==30980==    by 0x48DB6A3: g_task_return_now (gtask.c:1215)
==30980==    by 0x48DBF43: g_task_return.part.3 (gtask.c:1285)
==30980==    by 0x4918885: g_dbus_connection_call_done (gdbusconnection.c:5765)
==30980==    by 0x48DB6A3: g_task_return_now (gtask.c:1215)
==30980==    by 0x48DB6D7: complete_in_idle_cb (gtask.c:1229)
==30980==    by 0x4A20981: g_main_dispatch (gmain.c:3325)
==30980==    by 0x4A20981: g_main_context_dispatch (gmain.c:4016)
==30980==    by 0x4A20BEF: g_main_context_iterate.isra.23 (gmain.c:4092)
==30980==    by 0x4A20E33: g_main_loop_run (gmain.c:4290)
==30980==    by 0x2C5C9: main (main.c:509)