address: fix stack overflow bug when printing error

Commit 24e673a introduced a bug that cause the Error() function to
call itself recursively forever, causing a stack overflow. We explicitly
cast the error to its base type to avoid the recursion and add a small
test case that would've triggered the bug before.

address.go

@@ -23,15 +23,15 @@ import (
 type UnsupportedWitnessVerError byte
 
 func (e UnsupportedWitnessVerError) Error() string {
-	return fmt.Sprintf("unsupported witness version: %#x", e)
+	return fmt.Sprintf("unsupported witness version: %#x", byte(e))
 }
 
 // UnsupportedWitnessProgLenError describes an error where a segwit address
 // being decoded has an unsupported witness program length.
 type UnsupportedWitnessProgLenError int
 
 func (e UnsupportedWitnessProgLenError) Error() string {
-	return fmt.Sprintf("unsupported witness program length: %d", e)
+	return fmt.Sprintf("unsupported witness program length: %d", int(e))
 }
 
 var (

address_test.go

@@ -859,6 +859,14 @@ func TestAddresses(t *testing.T) {
 					test.name)
 				return
 			}
+		} else {
+			// If there is an error, make sure we can print it
+			// correctly.
+			errStr := err.Error()
+			if errStr == "" {
+				t.Errorf("%v: error was non-nil but message is"+
+					"empty: %v", test.name, err)
+			}
 		}
 
 		if !test.valid {