wallet: Lowerbound the rescan by going at most back to LNs origin

Repeated crashes could result in the `last_processed_block` variable being
pushed further and further into the past (in some cases going as far back as
scanning blocks from 2012...). This is a stop-gap solution that just lower
bounds the value to what is the first possible block we might be interested in
LN, until we have the 0-rescan fix I'm working on.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

wallet/wallet.c

@@ -826,6 +826,10 @@ u32 wallet_first_blocknum(struct wallet *w, u32 first_possible)
 	first_utxo = db_get_intvar(w->db, "last_processed_block", UINT32_MAX);
 #endif
 
+	/* Never go below the start of the Lightning Network */
+	if (first_utxo < first_possible)
+		first_utxo = first_possible;
+
 	if (first_utxo < first_channel)
 		return first_utxo;
 	else