Fix bug by casting to unsigned char.

Higher unciode codepoints where being read as negative char values.
We need to cast to unsigned char before comparing to 0x20 to check
for unprintable characters.

src/mainclient/shell.c

@@ -763,6 +763,7 @@ static int line() {
 
         switch (c) {
             default:
+                if ((unsigned char) c < 0x20) break;
                 if (insert(c, 1)) return -1;
                 break;
             case 1:     /* ctrl-a */