path/filepath: prevent infinite recursion on Windows on UNC input

This is a minimal fix to prevent this and
other possible future infinite recursion.
We can put in a proper fix for UNC in Go 1.8.

Updates #15879

Change-Id: I3653cf5891bab8511adf66fa3c1a1d8912d1a293
Reviewed-on: https://go-review.googlesource.com/23572
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/path/filepath/match.go

@@ -250,6 +250,11 @@ func Glob(pattern string) (matches []string, err error) {
 		return glob(dir, file, nil)
 	}
 
+	// Prevent infinite recursion. See issue 15879.
+	if dir == pattern {
+		return nil, ErrBadPattern
+	}
+
 	var m []string
 	m, err = Glob(dir)
 	if err != nil {

src/path/filepath/match_test.go

@@ -159,6 +159,12 @@ func TestGlobError(t *testing.T) {
 	}
 }
 
+func TestGlobUNC(t *testing.T) {
+	// Just make sure this runs without crashing for now.
+	// See issue 15879.
+	Glob(`\\?\C:\*`)
+}
+
 var globSymlinkTests = []struct {
 	path, dest string
 	brokenLink bool