Fix crash on ListPathRequest with malformed prefix

When ListPathRequest is done by a gRPC client including a malformed prefix,
 the server would crash an invalid memory address reference.

This commit avoid the crash by checking whether the parseCIDR method returned
an error.

pkg/packet/bgp/bgp.go

@@ -9582,7 +9582,14 @@ func NewPrefixFromRouteFamily(afi uint16, safi uint8, prefixStr ...string) (pref
 	family := AfiSafiToRouteFamily(afi, safi)
 
 	f := func(s string) AddrPrefixInterface {
-		addr, net, _ := net.ParseCIDR(s)
+		addr, net, err := net.ParseCIDR(s)
+		if err != nil {
+			switch family {
+			case RF_IPv4_UC, RF_IPv4_MC:
+				return NewIPAddrPrefix(0, "")
+			}
+			return NewIPv6AddrPrefix(0, "")
+		}
 		len, _ := net.Mask.Size()
 		switch family {
 		case RF_IPv4_UC, RF_IPv4_MC:
@@ -9593,17 +9600,9 @@ func NewPrefixFromRouteFamily(afi uint16, safi uint8, prefixStr ...string) (pref
 
 	switch family {
 	case RF_IPv4_UC, RF_IPv4_MC:
-		if len(prefixStr) > 0 {
-			prefix = f(prefixStr[0])
-		} else {
-			prefix = NewIPAddrPrefix(0, "")
-		}
+		prefix = f(prefixStr[0])
 	case RF_IPv6_UC, RF_IPv6_MC:
-		if len(prefixStr) > 0 {
-			prefix = f(prefixStr[0])
-		} else {
-			prefix = NewIPv6AddrPrefix(0, "")
-		}
+		prefix = f(prefixStr[0])
 	case RF_IPv4_VPN:
 		prefix = NewLabeledVPNIPAddrPrefix(0, "", *NewMPLSLabelStack(), nil)
 	case RF_IPv6_VPN: