Merge pull request #99 from jamslinger/slice-out-of-bound

Don't panic in when slicing with index out of bound

filters/standard_filters.go

@@ -185,11 +185,17 @@ func AddStandardFilters(fd FilterDictionary) { //nolint: gocyclo
 	})
 	fd.AddFilter("sort_natural", sortNaturalFilter)
 	fd.AddFilter("slice", func(s string, start int, length func(int) int) string {
+		if len(s) == 0 {
+			return ""
+		}
 		ss := []rune(s)
 		n := length(1)
 		if start < 0 {
 			start = len(ss) + start
 		}
+		if start < 0 {
+			return ""
+		}
 		end := start + n
 		if end > len(ss) {
 			end = len(ss)

filters/standard_filters_test.go

@@ -111,6 +111,8 @@ Liquid" | slice: 0`, "L"},
 	{`"Liquid
 Liquid" | slice: 2, 4`, "quid"},
 	{`"Liquid" | slice: -3, 2`, "ui"},
+	{`"" | slice: 1`, ""},
+	{`"Liquid" | slice: -7`, ""},
 
 	{`"a/b/c" | split: '/' | join: '-'`, "a-b-c"},
 	{`"a/b/" | split: '/' | join: '-'`, "a-b"},