fix variables that werent being called

Signed-off-by: Jessica Frazelle <acidburn@docker.com>

rewritten from github.com/moby/moby 0e025b4bb16c0d4cc6b3f0c040713d061b9b051a

frontend/dockerfile/parser/parser.go

@@ -136,7 +136,7 @@ func Parse(rwc io.Reader) (*Node, error) {
 				}
 			}
 			if child == nil && line != "" {
-				line, child, err = parseLine(line)
+				_, child, err = parseLine(line)
 				if err != nil {
 					return nil, err
 				}