pgbench: fix segfault with empty sql file

Commit 1d0c3b3f introduced a bug that causes pgbench to crash if an
empty script file is specified.  Fix it by rejecting such files at
startup, which is the historical and intended behavior.

Reported-By: Jeff Janes
Discussion: https://www.postgresql.org/message-id/CAMkU=1zxKUbLPOt9hQWFp14pTc=V0cGo2GQBbn2GsK2Pu+8ZfA@mail.gmail.com

src/bin/pgbench/pgbench.c

@@ -2686,7 +2686,8 @@ findBuiltin(const char *name, char **desc)
 static void
 addScript(const char *name, Command **commands)
 {
-	if (commands == NULL)
+	if (commands == NULL ||
+		commands[0] == NULL)
 	{
 		fprintf(stderr, "empty command list for script \"%s\"\n", name);
 		exit(1);