ash: parser: Fix incorrect eating of backslash newlines

ash: parser: Fix incorrect eating of backslash newlines

Keeping up with upstream (in our case, 'before patch' code is not buggy).
Upstream commit:

    Date: Fri, 11 May 2018 23:41:25 +0800
    parser: Fix incorrect eating of backslash newlines

    With the introduction of synstack->syntax, a number of references
    to the syntax variable was missed during the conversion.  This
    causes backslash newlines to be incorrectly removed in single
    quote context.

    This patch also combines these calls into a new helper function
    pgetc_top.

    Fixes: ab1cecb40478 ("parser: Add syntax stack for recursive...")
    Reported-by: Leah Neukirchen <leah@vuxu.org>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>