Fix error with trailing escape char

Created by: js6pak

Summary

Fixes error with trailing escape char (\).

Details

Currently when you try adding \ at the end of argument following exception will be thrown:

System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at DSharpPlus.CommandsNext.CommandsNextUtilities.ExtractNextArgument(String str, Int32& startPos)
   at DSharpPlus.CommandsNext.CommandsNextUtilities.BindArguments(CommandContext ctx, Boolean ignoreSurplus)
   at DSharpPlus.CommandsNext.Command.ExecuteAsync(CommandContext ctx)