Fix a longstanding bug, from Otto Moerbeck:

if we're reducing a rule that has an empty
right hand side and the yacc stackpointer is pointing at the very
end of the allocated stack, we end up accessing the stack out of
bounds by the implicit $$ = $1 action

Obtained from:	OpenBSD

usr.bin/yacc/skeleton.c

@@ -60,6 +60,7 @@ __FBSDID("$FreeBSD$");
 const char *banner[] =
 {
     "#include <stdlib.h>",
+    "#include <string.h>",
     "#ifndef lint",
     "#ifdef __unused",
     "__unused",
@@ -331,7 +332,10 @@ const char *body[] =
     "                YYPREFIX, yystate, yyn, yyrule[yyn]);",
     "#endif",
     "    yym = yylen[yyn];",
-    "    yyval = yyvsp[1-yym];",
+    "    if (yym)",
+    "        yyval = yyvsp[1-yym];",
+    "    else",
+    "    memset(&yyval, 0, sizeof yyval);",
     "    switch (yyn)",
     "    {",
     0