Crash happens when we try to propagate an error but no plugin_api is actually available.

Program received signal SIGSEGV, Segmentation fault.
_mcd_plugin_request_dup_denial (self=0x0) at plugin-request.c:261
261	  if (self->domain == 0)
(gdb) bt full
#0  _mcd_plugin_request_dup_denial (self=0x0) at plugin-request.c:261
No locals.
#1  0x00d73ce8 in _mcd_account_proceed_with_request (account=0x95b2008, channel=0x95af588) at mcd-account-requests.c:237
        plugin_api = <value optimized out>
        error = 0x0
        mini_plugins = 0x0
        __PRETTY_FUNCTION__ = "_mcd_account_proceed_with_request"
#2  0x00d92e43 in channel_request_proceed (iface=0x95af588, context=0x95c4000) at mcd-channel.c:1529
        self = 0x95af588
        __PRETTY_FUNCTION__ = "channel_request_proceed"
#3  0x001d1033 in tp_svc_channel_request_proceed (self=0x95af588, context=0x95c4000) at _gen/tp-svc-channel-request.c:65
        impl = <value optimized out>
#4  0x0028f798 in g_cclosure_marshal_VOID__POINTER () from /lib/libgobject-2.0.so.0
No symbol table info available.
#5  0x005ac56f in ?? () from /usr/lib/libdbus-glib-1.so.2
No symbol table info available.
#6  0x005acb31 in ?? () from /usr/lib/libdbus-glib-1.so.2
No symbol table info available.
#7  0x0024f751 in ?? () from /lib/libdbus-1.so.3
No symbol table info available.
#8  0x0024215a in dbus_connection_dispatch () from /lib/libdbus-1.so.3
No symbol table info available.
#9  0x005a920e in ?? () from /usr/lib/libdbus-glib-1.so.2
No symbol table info available.
#10 0x002f3f88 in g_main_context_dispatch () from /lib/libglib-2.0.so.0
No symbol table info available.
#11 0x002f78b8 in ?? () from /lib/libglib-2.0.so.0
No symbol table info available.
#12 0x002f7d2f in g_main_loop_run () from /lib/libglib-2.0.so.0
No symbol table info available.
#13 0x00d93ea0 in mcd_service_run (self=0x95aa410) at mcd-service.c:195
No locals.
#14 0x08048b17 in main (argc=1, argv=0xbf8a2434) at mc-server.c:78
        mcd = 0x95aa410