ash: parser: Fix old-style command substitution here-document crash

Upstream commit:

    Date: Fri, 29 Mar 2019 13:49:59 +0800
    parser: Fix old-style command substitution here-document crash

    ... This is caused by the recent change to
    save/restore here-docment list around command substitutions.  In
    doing so we must finish existing here-documents prior to restoring
    the old here-document list.  This is done for new-style command
    substitutions but not for old-style.

    This patch fixes it by doing it for both.

    Fixes: 51e2d88d6e51 ("parser: Save/restore here-documents in...")
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash.c

@@ -12877,9 +12877,9 @@ parsebackq: {
 		if (readtoken() != TRP)
 			raise_error_unexpected_syntax(TRP);
 		setinputstring(nullstr);
-		parseheredoc();
 	}
 
+	parseheredoc();
 	heredoclist = saveheredoclist;
 
 	(*nlpp)->n = n;