hush: fix another corner case with backslashes in heredocs

function                                             old     new   delta
parse_stream                                        2395    2432     +37

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>

shell/hush_test/hush-misc/heredoc_backslash1.tests

@@ -7,6 +7,8 @@ echo Quoted heredoc:
 cat <<"EOF1"
 a\
 	b
+a\\
+	b
  123456 -$a-\t-\\-\"-\'-\`-\--\z-\*-\?-
 	-$a-\t-\\-\"-\'-\`-\--\z-\*-\?-
 c\
@@ -17,6 +19,8 @@ echo Unquoted heredoc:
 cat <<EOF2
 a\
 	b
+a\\
+	b
  123456 -$a-\t-\\-\"-\'-\`-\--\z-\*-\?-
 	-$a-\t-\\-\"-\'-\`-\--\z-\*-\?-
 c\
@@ -28,6 +32,8 @@ echo Quoted -heredoc:
 cat <<-"EOF3"
 a\
 	b
+a\\
+	b
  123456 -$a-\t-\\-\"-\'-\`-\--\z-\*-\?-
 	-$a-\t-\\-\"-\'-\`-\--\z-\*-\?-
 c\
@@ -39,6 +45,8 @@ echo Unquoted -heredoc:
 cat <<-EOF4
 a\
 	b
+a\\
+	b
  123456 -$a-\t-\\-\"-\'-\`-\--\z-\*-\?-
 	-$a-\t-\\-\"-\'-\`-\--\z-\*-\?-
 c\