accounts/abi: remove check for len%32==0 when unpacking events (#15670)

This change inlines the logic of bytesAreProper at its sole
callsite, ABI.Unpack, and applies the multiple-of-32 test only in
the case of unpacking methods. Event data is not required to be a
multiple of 32 bytes long.

accounts/abi/abi.go

@@ -74,13 +74,17 @@ func (abi ABI) Pack(name string, args ...interface{}) ([]byte, error) {
 
 // Unpack output in v according to the abi specification
 func (abi ABI) Unpack(v interface{}, name string, output []byte) (err error) {
-	if err = bytesAreProper(output); err != nil {
-		return err
+	if len(output) == 0 {
+		return fmt.Errorf("abi: unmarshalling empty output")
 	}
+
 	// since there can't be naming collisions with contracts and events,
 	// we need to decide whether we're calling a method or an event
 	var unpack unpacker
 	if method, ok := abi.Methods[name]; ok {
+		if len(output)%32 != 0 {
+			return fmt.Errorf("abi: improperly formatted output")
+		}
 		unpack = method
 	} else if event, ok := abi.Events[name]; ok {
 		unpack = event