instructions: avoid allocating memory and processing location for nil

Nil errors get ignored by WithLocation but the structures that
create slices with location information still need to be created
before they get discarded.

Signed-off-by: Tonis Tiigi <[email protected]>

Author: tonistiigi

frontend/dockerfile/instructions/parse.go

@@ -73,7 +73,9 @@ func ParseInstruction(node *parser.Node) (v interface{}, err error) {
 // ParseInstruction converts an AST to a typed instruction (either a command or a build stage beginning when encountering a `FROM` statement)
 func ParseInstructionWithLinter(node *parser.Node, lint *linter.Linter) (v interface{}, err error) {
 	defer func() {
-		err = parser.WithLocation(err, node.Location())
+		if err != nil {
+			err = parser.WithLocation(err, node.Location())
+		}
 	}()
 	req := newParseRequestFromNode(node)
 	switch strings.ToLower(node.Value) {