Correctly handle files that contain no slides

Otherwise, on reloading sent would segfault. Move the check-logic to
load() rather than doing it in main() to prevent other misuses of it in
the future.

sent.c

@@ -457,6 +457,9 @@ load(FILE *fp)
 		if (!p)
 			break;
 	}
+
+	if (!slidecount)
+		die("sent: No slides in file");
 }
 
 void
@@ -695,9 +698,6 @@ main(int argc, char *argv[])
 	load(fp);
 	fclose(fp);
 
-	if (!slidecount)
-		usage();
-
 	xinit();
 	run();