Fix command substitutions run on the same line as a here-doc

[JohnoKing]

When a command substitution is run on the same line as a here-document, a syntax error occurs due to a regression introduced in ksh93u+ 2011-04-15:

$ cat ./foo.sh
true << EOF; true $(true)
EOF
$ ksh ./foo.sh
./foo.sh: syntax error at line 1: `<<EOF' here-document not contained within command substitution

The regression is caused by the following error check, which was added to fix a bug with here-documents in command substitutions:

ksh/src/cmd/ksh93/sh/lex.c

Lines 1692 to 1693 in f207cd5

	if(lp->heredoc)
	errormsg(SH_DICT,ERROR_exit(SYNBAD),e_lexsyntax5,lp->sh->inlineno,lp->heredoc->ioname);

# A syntax error should occur because the here-document isn't completed
# in the command substitution, but in versions of ksh prior to ksh93u+
# 2011-04-15 this isn't an error.
$(true << EOF)
EOF

The bugfix (which was backported from ksh93v- 2013-10-10-alpha) stops the syntax error from occurring when a here-document is not inside of a command substitution. It should only be a syntax error when a here-document inside of a command substitution isn't completed before the ending ')'.