vppinfra: loop counter off by 1 in search_free_list()

vppinfra: loop counter off by 1 in search_free_list()

In search_free_list(), we have this do while loop.
	do
	  {
	    l--;
	    f_index = h->free_lists[b][l];
	    f = elt_at (h, f_index);
	    f_size = heap_elt_size (v, f);
	    if ((s = f_size - size) >= 0)
	      break;
	  }
	while (l >= 0);

When (l == 0), we still go back up to execute l--. Then l become -1. The
next statement is we index h->free_lists[b][-1]. After that, elt_at() would
probably cause a crash in the ASSERT.

Type: fix
Ticket: VPPSUPP-63

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: I617d122aa221cfdfe38f8be50f4e0f0e76e11bb5