Fix weird printing to console that sometimes happened with listImages

It sometimes happened when changing image_* cvars for compression and
then vid_restarting and then running listImages.
In one case the whole process hung and len was a ridiculously big number
=> most probably the (unsigned!) size_t overflowed by subtracting a
number > len (most likely from input_field.GetCursor()).

So I just made it a plain signed int.
I couldn't reproduce the issue anymore after doing this change.

Author: DanielGibson

neo/sys/posix/posix_main.cpp

@@ -828,7 +828,9 @@ void tty_Show() {
 	input_hide--;
 	if ( input_hide == 0 ) {
 		char *buf = input_field.GetBuffer();
-		size_t len = strlen(buf);
+		// DG: it happened (not sure how) that len became very big,
+		//     most likely because of an overflow (underflow?) so I made it signed
+		int len = strlen(buf);
 		if ( len < 1 )
 			return;