addrmgr: Fix AddressCache to not include nils

allAddr was being allocated with counters instead of the actual size
of the address map.  This led to the possibility of including nils
in the returned slice, which resulted in a panic.

addrmgr/addrmanager.go

@@ -643,19 +643,19 @@ func (a *AddrManager) NeedMoreAddresses() bool {
 func (a *AddrManager) AddressCache() []*wire.NetAddress {
 	a.mtx.Lock()
 	defer a.mtx.Unlock()
-	if a.nNew+a.nTried == 0 {
+
+	addrIndexLen := len(a.addrIndex)
+	if addrIndexLen == 0 {
 		return nil
 	}
 
-	allAddr := make([]*wire.NetAddress, a.nNew+a.nTried)
-	i := 0
+	allAddr := make([]*wire.NetAddress, 0, addrIndexLen)
 	// Iteration order is undefined here, but we randomise it anyway.
 	for _, v := range a.addrIndex {
-		allAddr[i] = v.na
-		i++
+		allAddr = append(allAddr, v.na)
 	}
 
-	numAddresses := len(allAddr) * getAddrPercent / 100
+	numAddresses := addrIndexLen * getAddrPercent / 100
 	if numAddresses > getAddrMax {
 		numAddresses = getAddrMax
 	}
@@ -664,12 +664,12 @@ func (a *AddrManager) AddressCache() []*wire.NetAddress {
 	// `numAddresses' since we are throwing the rest.
 	for i := 0; i < numAddresses; i++ {
 		// pick a number between current index and the end
-		j := rand.Intn(len(allAddr)-i) + i
+		j := rand.Intn(addrIndexLen-i) + i
 		allAddr[i], allAddr[j] = allAddr[j], allAddr[i]
 	}
 
 	// slice off the limit we are willing to share.
-	return allAddr[:numAddresses]
+	return allAddr[0:numAddresses]
 }
 
 // reset resets the address manager by reinitialising the random source