ash: expand: Fix trailing newlines processing in backquote expanding

ash: expand: Fix trailing newlines processing in backquote expanding

Upstream commit:

    Date: Mon, 29 Apr 2019 19:13:37 +0500
    expand: Fix trailing newlines processing in backquote expanding

    According to POSIX.1-2008 we should remove newlines only at the end of
    the substitution. Newlines-only substitions causes dash to remove
    newlines before beggining of the substitution. The following code:

        cat <<END
        1
        $(echo "")
        2
        END

    prints "1<newline>2" instead of expected "1<newline><newline>2".

    This patch fixes trailing newlines processing in backquote expanding.

    Signed-off-by: Nikolai Merinov <n.merinov@inango-systems.com>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>