sh: Fix a crash if a heredoc was not properly ended and parsing continued.

Example (in interactive mode):
  cat <<EOF && )
The next command typed caused sh to segfault, because the state for the here
document was not reset.

Like parser_temp, this uses the fact that the parser is not re-entered.

bin/sh/parser.c

@@ -203,6 +203,7 @@ parsecmd(int interact)
 	 * which could happen if we add command substitution on PS1/PS2.
 	 */
 	parser_temp_free_all();
+	heredoclist = NULL;
 
 	tokpushback = 0;
 	doprompt = interact;

tools/regression/bin/sh/parser/heredoc6.0

@@ -0,0 +1,5 @@
+# $FreeBSD$
+
+r=
+! command eval ": <<EOF; )" 2>/dev/null; command eval : hi \${r:=0}
+exit ${r:-3}