allow generation of gentxs with empty memo field

Let gentx generation continue when gaiad is unable to retrieve
machine's external IP and print a warning message.

Closes: #3424

PENDING.md

@@ -51,6 +51,7 @@ IMPROVEMENTS
   * [\#3418](https://github.com/cosmos/cosmos-sdk/issues/3418) Add vesting account
   genesis validation checks to `GaiaValidateGenesisState`.
   * [\#3420](https://github.com/cosmos/cosmos-sdk/issues/3420) Added maximum length to governance proposal descriptions and titles
+  * [\#3424](https://github.com/cosmos/cosmos-sdk/issues/3424) Allow generation of gentxs with empty memo field.
 
 * SDK
   * \#3435 Test that store implementations do not allow nil values

cmd/gaia/init/gentx.go

@@ -61,9 +61,11 @@ following delegation and commission default parameters:
 			if err != nil {
 				return err
 			}
+
 			ip, err := server.ExternalIP()
 			if err != nil {
-				return err
+				fmt.Fprintf(os.Stderr, "couldn't retrieve an external IP, "+
+					"consequently the tx's memo field will be unset: %s", err)
 			}
 
 			genDoc, err := LoadGenesisDoc(cdc, config.GenesisFile())