regexp: don't return non-nil *Regexp if there is an error.

R=gri
CC=golang-dev
https://golang.org/cl/787041

src/pkg/regexp/regexp.go

@@ -664,6 +664,9 @@ func Compile(str string) (regexp *Regexp, error os.Error) {
 	regexp.expr = str
 	regexp.inst = new(vector.Vector)
 	error = regexp.doParse()
+	if error != nil {
+		regexp = nil
+	}
 	return
 }