bug#66713: Expr substr on plus symbol

[petabaud51]

Greetings,
I'm not sure if this is the intended UNIX/POSIX behaviour, but on:
< expr substr a 1 2
, I get:
> a
, which is right, but on:
< expr substr + 1 2
I get:
> expr: syntax error: missing argument after ‘2’

On expr "$line_of_text" 1 2, this error is thrown if the line is a 
simple '+'.
A real-world scenario is getting the first character of each line in 
bulk, crashing if the line is '+'.

< expr --version
> expr (GNU coreutils) 8.32

--