Fix decodeSlice to check input type before calling its Len method

mapstructure.go

diff --git a/mapstructure.go b/mapstructure.go
index 0c23d8b..62fe1be 100644
--- a/mapstructure.go
+++ b/mapstructure.go
@@ -470,16 +470,13 @@
 	dataValKind := dataVal.Kind()
 	valType := val.Type()
 	valElemType := valType.Elem()
-
-	// Make a new slice to hold our result, same size as the original data.
 	sliceType := reflect.SliceOf(valElemType)
-	valSlice := reflect.MakeSlice(sliceType, dataVal.Len(), dataVal.Len())
 
 	// Check input type
 	if dataValKind != reflect.Array && dataValKind != reflect.Slice {
 		// Accept empty map instead of array/slice in weakly typed mode
 		if d.config.WeaklyTypedInput && dataVal.Kind() == reflect.Map && dataVal.Len() == 0 {
-			val.Set(valSlice)
+			val.Set(reflect.MakeSlice(sliceType, 0, 0))
 			return nil
 		} else {
 			return fmt.Errorf(
@@ -487,6 +484,9 @@
 		}
 	}
 
+	// Make a new slice to hold our result, same size as the original data.
+	valSlice := reflect.MakeSlice(sliceType, dataVal.Len(), dataVal.Len())
+
 	// Accumulate any errors
 	errors := make([]string, 0)
 

mapstructure_test.go

diff --git a/mapstructure_test.go b/mapstructure_test.go
index e82d508..3147f55 100644
--- a/mapstructure_test.go
+++ b/mapstructure_test.go
@@ -593,6 +593,21 @@
 	testSliceInput(t, inputStringSlicePointer, outputStringSlice)
 }
 
+func TestInvalidSlice(t *testing.T) {
+	t.Parallel()
+
+	input := map[string]interface{}{
+		"vfoo": "foo",
+		"vbar": 42,
+	}
+
+	result := Slice{}
+	err := Decode(input, &result)
+	if err == nil {
+		t.Errorf("expected failure")
+	}
+}
+
 func TestSliceOfStruct(t *testing.T) {
 	t.Parallel()