Don't let trailing empty lines overwrite the result of the last command with 0.

This affects the built-ins eval, fc, and trap and also the string passed to sh
with the -c option.

Submitted by:	Jilles Tjoelker

bin/sh/eval.c

@@ -166,7 +166,8 @@ evalstring(char *s)
 	setstackmark(&smark);
 	setinputstring(s, 1);
 	while ((n = parsecmd(0)) != NEOF) {
-		evaltree(n, 0);
+		if (n != NULL)
+			evaltree(n, 0);
 		popstackmark(&smark);
 	}
 	popfile();