Limit callbacks without updates to 1 per 30s; ensure slist is freed even if there isn't a callback

[jtbr]

Could just be my network, but I can get callbacks many times in a single second with an apparently empty slist.

[23:49:06.987466] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:06.987654] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:06.987706] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.058365] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.267265] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.267363] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.268833] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.286907] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.287008] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.287062] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.287113] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.287158] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.293361] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.293506] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.301853] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.523187] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.523277] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.528106] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.712744] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.712849] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.712901] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:07.716325] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:08.006569] mDNSsearchCallback:718 ----------------- round ------------------
[23:49:08.006688] mDNSsearchCallback:718 ----------------- round ------------------

This change will limit that to no more than one per 30 seconds. Minor optimization, but still, seems worth having. It also ensures that slist is freed in the unusual case that there is no callback provided.