tweaks: slightly optimize an allocation in display_string()

Instead of allocating enough space to convert the entire passed string,
just allocate space for converting the part that will be converted --
that is: starting from start_index.  This still allocates far too much
(if the passed string is very long and its tail part won't fit on the
screen), but it's better than before.

src/winio.c

@@ -1823,15 +1823,15 @@ char *display_string(const char *buf, size_t start_col, size_t span,
 
     assert(column <= start_col);
 
-    /* Allocate enough space to hold the entire converted buffer. */
-    converted = charalloc(strlen(buf) * (mb_cur_max() + tabsize) + 1);
-
     index = 0;
 #ifdef USING_OLD_NCURSES
     seen_wide = FALSE;
 #endif
     buf += start_index;
 
+    /* Allocate enough space for converting the relevant part of the line. */
+    converted = charalloc(strlen(buf) * (mb_cur_max() + tabsize) + 1);
+
     /* If the first character starts before the left edge, or would be
      * overwritten by a "$" token, then show spaces instead. */
     if (*buf != '\0' && *buf != '\t' && (column < start_col ||