buffer: check sudo binary exists before creating backup

Verify the sudo command is available via exec.LookPath() before
creating the backup in safeWrite(). This avoids bogus backup files
when the sudo binary is not found.

Signed-off-by: Nilton Perim Neto <niltonperimneto@gmail.com>

Author: niltonperimneto

internal/buffer/save.go

@@ -238,6 +238,13 @@ func (b *Buffer) saveToFile(filename string, withSudo bool, autoSave bool) error
 		return errors.New("Cannot save scratch buffer")
 	}
 
+	if withSudo {
+		_, err := exec.LookPath(config.GlobalSettings["sucmd"].(string))
+		if err != nil {
+			return err
+		}
+	}
+
 	if !autoSave && b.Settings["rmtrailingws"].(bool) {
 		for i, l := range b.lines {
 			leftover := util.CharacterCount(bytes.TrimRightFunc(l.data, unicode.IsSpace))