Fix possible panics in slice type assertion

Before accessing the slices `.Value()` we should type check that it is
that actual type, otherwise we will end-up in a panic.

Signed-off-by: Sascha Grunert <sgrunert@suse.com>

flag_float64_slice.go

@@ -155,11 +155,9 @@ func (c *Context) Float64Slice(name string) []float64 {
 func lookupFloat64Slice(name string, set *flag.FlagSet) []float64 {
 	f := set.Lookup(name)
 	if f != nil {
-		parsed, err := (f.Value.(*Float64Slice)).Value(), error(nil)
-		if err != nil {
-			return nil
+		if slice, ok := f.Value.(*Float64Slice); ok {
+			return slice.Value()
 		}
-		return parsed
 	}
 	return nil
 }

flag_int64_slice.go

@@ -151,11 +151,9 @@ func (c *Context) Int64Slice(name string) []int64 {
 func lookupInt64Slice(name string, set *flag.FlagSet) []int64 {
 	f := set.Lookup(name)
 	if f != nil {
-		parsed, err := (f.Value.(*Int64Slice)).Value(), error(nil)
-		if err != nil {
-			return nil
+		if slice, ok := f.Value.(*Int64Slice); ok {
+			return slice.Value()
 		}
-		return parsed
 	}
 	return nil
 }

flag_int_slice.go

@@ -165,11 +165,9 @@ func (c *Context) IntSlice(name string) []int {
 func lookupIntSlice(name string, set *flag.FlagSet) []int {
 	f := set.Lookup(name)
 	if f != nil {
-		parsed, err := (f.Value.(*IntSlice)).Value(), error(nil)
-		if err != nil {
-			return nil
+		if slice, ok := f.Value.(*IntSlice); ok {
+			return slice.Value()
 		}
-		return parsed
 	}
 	return nil
 }

flag_string_slice.go

@@ -149,11 +149,9 @@ func (c *Context) StringSlice(name string) []string {
 func lookupStringSlice(name string, set *flag.FlagSet) []string {
 	f := set.Lookup(name)
 	if f != nil {
-		parsed, err := (f.Value.(*StringSlice)).Value(), error(nil)
-		if err != nil {
-			return nil
+		if slice, ok := f.Value.(*StringSlice); ok {
+			return slice.Value()
 		}
-		return parsed
 	}
 	return nil
 }