Prevent overflow beyond the end of string in visible_length()

Bug-Debian: https://github.com/tats/w3m/issues/73

table.c

@@ -428,8 +428,9 @@ visible_length(char *str)
     Str tagbuf = Strnew();
     char *t, *r2;
     int amp_len = 0;
+    char *strz = str + strlen(str);
 
-    while (*str) {
+    while (str < strz) {
 	prev_status = status;
 	if (next_status(*str, &status)) {
 #ifdef USE_M17N