exp/regexp/syntax: fix invalid input parser crash

Reported by Kyle Lemons.

R=r
CC=golang-dev
https://golang.org/cl/4992045
diff --git a/src/pkg/exp/regexp/syntax/parse_test.go b/src/pkg/exp/regexp/syntax/parse_test.go
index 5c8107c..f20276c 100644
--- a/src/pkg/exp/regexp/syntax/parse_test.go
+++ b/src/pkg/exp/regexp/syntax/parse_test.go
@@ -482,6 +482,7 @@
 	"a?*",
 	"a+*",
 	"a{1}*",
+	".{1}{2}.{3}",
 }
 
 func TestParseInvalidRegexps(t *testing.T) {