ash: expand: Fix multiple issues with EXP_DISCARD in evalvar

ash: expand: Fix multiple issues with EXP_DISCARD in evalvar

Upstream commit:

    Date: Wed, 12 Sep 2018 14:27:16 +0800
    expand: Fix multiple issues with EXP_DISCARD in evalvar

    The commit 3cd538634f71538370f5af239f342aec48b7470b broke parameter
    expansion in multiple ways because the EXP_DISCARD flag wasn't set
    or tested for various cases:

            $ src/dash -c 'var=; echo ${var:+nonempty}'
            nonempty
            $ src/dash -u -c 'unset foo bar; echo ${foo+${bar}}'
            dash: 1: bar: parameter not set
            $ src/dash -c 'foo=bar; echo ${foo=BUG}; echo $foo'
            barBUG
            bar
            $

    This patch fixes them by introducing a new discard variable that
    tracks whether the extra word should be discarded or not when it
    is parsed.

    Reported-by: Martijn Dekker <martijn@inlv.org>
    Fixes: 3cd538634f71 ("expand: Do not reprocess data when...")
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
    Reported-by: Martijn Dekker <martijn@inlv.org>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>