Rework Torrent pending requests assertions

Should provide more info, and sooner for #249.

connection.go

@@ -1369,7 +1369,15 @@ func (c *connection) deleteRequest(r request) bool {
 		return false
 	}
 	delete(c.requests, r)
-	c.t.pendingRequests[r]--
+	pr := c.t.pendingRequests
+	pr[r]--
+	n := pr[r]
+	if n == 0 {
+		delete(pr, r)
+	}
+	if n < 0 {
+		panic(n)
+	}
 	c.updateRequests()
 	return true
 }

torrent.go

@@ -1231,10 +1231,8 @@ func (t *Torrent) deleteConnection(c *connection) (ret bool) {
 }
 
 func (t *Torrent) assertNoPendingRequests() {
-	for _, num := range t.pendingRequests {
-		if num != 0 {
-			panic(num)
-		}
+	if len(t.pendingRequests) != 0 {
+		panic(t.pendingRequests)
 	}
 }