bug#22045: expr substr returns with an error code 1 when the substring starts with 0

[Matthias Klose]

found with coreutils-8.23, expr substr returns with an error code when the 
substring consists of only '0' characters, and the match is started at position 1.

$ expr substr 00001234 3 4; echo $?
0012
0
$ expr substr 1234 1 2; echo $?
12
0

$ expr substr 00001234 1 2; echo $?
00
1
$ expr substr 00001234 1 4; echo $?
0000
1
$ expr substr 00001234 1 5; echo $?
00001
0