sh: Fix crash due to uninitialized here-document.

If an ; or & token was followed by an EOF token, pending here-documents were
left uninitialized. Execution would crash, either in the main shell process
for literal here-documents or in a child process for expanded
here-documents. In the latter case the problem is hard to detect apart from
the core dumps and log messages.

Side effect: slightly different retries on inputs where EOF is not
persistent.

Note that tools/regression/bin/sh/parser/heredoc6.0 still causes a similar
crash in a child process. The text passed to eval is malformed and should be
rejected.

bin/sh/parser.c

@@ -269,6 +269,9 @@ list(int nlflag)
 				parseheredoc();
 				if (nlflag)
 					return n1;
+			} else if (tok == TEOF && nlflag) {
+				parseheredoc();
+				return n1;
 			} else {
 				tokpushback++;
 			}

tools/regression/bin/sh/parser/heredoc7.0

@@ -0,0 +1,19 @@
+# $FreeBSD$
+
+# Some of these created malformed parse trees with null pointers for here
+# documents, causing the here document writing process to segfault.
+eval ': <<EOF'
+eval ': <<EOF;'
+eval '`: <<EOF`'
+eval '`: <<EOF;`'
+eval '`: <<EOF`;'
+eval '`: <<EOF;`;'
+
+# Some of these created malformed parse trees with null pointers for here
+# documents, causing sh to segfault.
+eval ': <<\EOF'
+eval ': <<\EOF;'
+eval '`: <<\EOF`'
+eval '`: <<\EOF;`'
+eval '`: <<\EOF`;'
+eval '`: <<\EOF;`;'