Fix potion command not supporting infinite durations (#6236)

Minecraft understands infinite durations to be `-1` and only `-1`. Our
second -> tick conversion was changing this to `-20` causing a potion
command with a `-1` duration to end up as `-20` and only causing the
potion to occur for 1 tick.

Fixes #5558

Author: JRoy

Essentials/src/main/java/com/earth2me/essentials/MetaItemStack.java

@@ -549,7 +549,8 @@ public void addPotionMeta(final CommandSource sender, final boolean allowShortNa
             } else if (split[0].equalsIgnoreCase("duration") || (allowShortName && split[0].equalsIgnoreCase("d"))) {
                 if (NumberUtil.isInt(split[1])) {
                     validPotionDuration = true;
-                    duration = Integer.parseInt(split[1]) * 20; //Duration is in ticks by default, converted to seconds
+                    final int parsed = Integer.parseInt(split[1]);
+                    duration = parsed == -1 ? -1 : parsed * 20; //Duration is in ticks by default, converted to seconds
                 } else {
                     throw new TranslatableException("invalidPotionMeta", split[1]);
                 }