SHLVL is a random string (uninitialized string in C) when its true value is "1000"

[ziyunfei]

$ SHLVL=998 bash -c 'echo $SHLVL'
999
$ SHLVL=999 bash -c 'echo $SHLVL'

$ SHLVL=999 bash -c 'echo -n "$SHLVL" | hexdump'
0b 01
$ SHLVL=999 bash -c 'echo -n "$SHLVL" | hexdump'
0f 01
$ SHLVL=999 bash -c 'echo -n "$SHLVL" | hexdump'
04 01
$ SHLVL=1000 bash -c 'echo $SHLVL'
bash: warning: shell level (1001) too high, resetting to 1
1