fix(routing): defensive nil checks for multiaddr handling

Belt-and-suspenders defense against corrupted address data:
- types/ipfs.go: Multiaddr.MarshalJSON returns null for nil
- contentrouter: filters nil addresses when building AddrInfo

Should help with ipfs/kubo#11116

Author: lidel

routing/http/contentrouter/contentrouter.go

@@ -147,9 +147,13 @@ func readProviderResponses(ctx context.Context, iter iter.ResultIter[types.Recor
 				continue
 			}
 
+			// Filter nil addresses as a defensive measure against corrupted data.
+			// See: https://github.com/ipfs/kubo/issues/11116
 			var addrs []multiaddr.Multiaddr
 			for _, a := range result.Addrs {
-				addrs = append(addrs, a.Multiaddr)
+				if a.Multiaddr != nil {
+					addrs = append(addrs, a.Multiaddr)
+				}
 			}
 
 			select {
@@ -175,9 +179,13 @@ func readProviderResponses(ctx context.Context, iter iter.ResultIter[types.Recor
 				continue
 			}
 
+			// Filter nil addresses as a defensive measure against corrupted data.
+			// See: https://github.com/ipfs/kubo/issues/11116
 			var addrs []multiaddr.Multiaddr
 			for _, a := range result.Addrs {
-				addrs = append(addrs, a.Multiaddr)
+				if a.Multiaddr != nil {
+					addrs = append(addrs, a.Multiaddr)
+				}
 			}
 
 			select {

routing/http/types/ipfs.go

@@ -27,6 +27,16 @@ func (c *CID) UnmarshalJSON(b []byte) error {
 
 type Multiaddr struct{ multiaddr.Multiaddr }
 
+// MarshalJSON returns null for nil Multiaddr as a defensive measure.
+// This prevents panics if corrupted data contains nil addresses.
+// See: https://github.com/ipfs/kubo/issues/11116
+func (m Multiaddr) MarshalJSON() ([]byte, error) {
+	if m.Multiaddr == nil {
+		return []byte("null"), nil
+	}
+	return json.Marshal(m.Multiaddr.String())
+}
+
 func (m *Multiaddr) UnmarshalJSON(b []byte) error {
 	var s string
 	err := json.Unmarshal(b, &s)