Fix check when char is unsigned (#665)

If char is unsigned (as on ARM) subtracting a larger number would
result in a wrap around, not a negative value.

  src/nnn.c: In function 'readpipe':
  src/nnn.c:4325:11: warning: comparison is always false due to limited
  range of data type [-Wtype-limits]

     if (ctx < 0 || ctx > CTX_MAX)
             ^

src/nnn.c

@@ -4326,9 +4326,11 @@ static void readpipe(int fd, char **path, char **lastname, char **lastdir)
 
 	if (g_buf[0] == '+')
 		ctx = (char)(get_free_ctx() + 1);
+	else if (g_buf[0] < '0')
+		return;
 	else {
 		ctx = g_buf[0] - '0';
-		if (ctx < 0 || ctx > CTX_MAX)
+		if (ctx > CTX_MAX)
 			return;
 	}