Forbid invalid combination of options in pg_basebackup.

Commit 56c7d8d455 allowed pg_basebackup
to stream WAL in tar mode. But there is the restriction that WAL
streaming in tar mode works only when the value - (dash) is not
specified as output directory. This means that the combination of
three options "-D -", "-F t" and "-X stream" is invalid. However,
previously, even when those options were specified at the same time,
pg_basebackup background process unexpectedly started streaming WAL.
And then it exited with an error.

This commit changes pg_basebackup so that it errors out on such
invalid combination of options at the beginning.

Reviewed by Magnus Hagander, and patch by me.

src/bin/pg_basebackup/pg_basebackup.c

@@ -2268,6 +2268,16 @@ main(int argc, char **argv)
 		exit(1);
 	}
 
+	if (format == 't' && streamwal && strcmp(basedir, "-") == 0)
+	{
+		fprintf(stderr,
+			_("%s: cannot stream transaction logs in tar mode to stdout\n"),
+				progname);
+		fprintf(stderr, _("Try \"%s --help\" for more information.\n"),
+				progname);
+		exit(1);
+	}
+
 	if (replication_slot && !streamwal)
 	{
 		fprintf(stderr,