btrfs-map-logical: segfaults when no output file is given

when no output file is given, info_file stays NULL and the following
fprintf segfaults. Default to stdout.

Signed-off-by: Arne Jansen <sensille@gmx.net>
Signed-off-by: Hugo Mills <hugo@carfax.org.uk>

btrfs-map-logical.c

@@ -173,6 +173,7 @@ int main(int ac, char **av)
 		exit(1);
 	}
 
+	info_file = stdout;
 	if (output_file) {
 		if (strcmp(output_file, "-") == 0) {
 			out_fd = 1;