filter: Fix crash for '**' pattern

changelog/unreleased/issue-3380

@@ -0,0 +1,7 @@
+Bugfix: Fix crash of `backup --exclude='**'`
+
+The exclude filter '**', which excludes all files, caused restic to crash. This
+has been fixed.
+
+https://github.com/restic/restic/issues/3380
+https://github.com/restic/restic/pull/3393

internal/filter/filter.go

@@ -166,6 +166,11 @@ func match(patterns Pattern, strs []string) (matched bool, err error) {
 		return true, nil
 	}
 
+	// an empty pattern never matches a non-empty path
+	if len(patterns) == 0 {
+		return false, nil
+	}
+
 	if len(patterns) <= len(strs) {
 		minOffset := 0
 		maxOffset := len(strs) - len(patterns)

internal/filter/filter_test.go

@@ -23,6 +23,7 @@ var matchTests = []struct {
 	{"*.go", "/foo/bar/test.go", true},
 	{"*.c", "/foo/bar/test.go", false},
 	{"*", "/foo/bar/test.go", true},
+	{"**", "/foo/bar/test.go", true},
 	{"foo*", "/foo/bar/test.go", true},
 	{"bar*", "/foo/bar/test.go", true},
 	{"/bar*", "/foo/bar/test.go", false},