From: r1w1s1 <r1w1s1@fastmail.com>
Date: Wed, 22 Apr 2026
Subject: Reject empty argument to -c flag

Without this check, `mtm -c ""` dereferences optarg[0] which is '\0',
making CTL('\0') == 0 the command prefix. Every NUL byte in PTY input
then triggers command mode. Ignore empty strings and keep the default.
---
--- a/mtm.c
+++ b/mtm.c
@@ -665,7 +665,7 @@ main(int argc, char **argv)
 
     int c = 0;
     while ((c = getopt(argc, argv, "c:T:t:")) != -1) switch (c){
-        case 'c': commandkey = CTL(optarg[0]);      break;
+        case 'c': if (optarg[0]) commandkey = CTL(optarg[0]); break;
         case 'T': setenv("TERM", optarg, 1);        break;
         case 't': term = optarg;                    break;
         default:  quit(EXIT_FAILURE, USAGE);        break;