Fix several buffer and integer overflow issues discovered by Timo Sirainen.

Fix several buffer and integer overflow issues discovered by Timo Sirainen.

tvbuff.c:

  Lots of existing code assumes that you can safely do the following:

    #define MAX_BUF 64
    guint8 *buf[MAX_BUF];
    ...

    tvb_get_nstringz0 (tvb, offset, MAX_BUF, buf, &bytes_copied);

  In reality, tvb_get_nstringz*() can potentially write one byte past
  "buf".  Modify _tvb_get_nstringz() not to do that.

packet-ppp.c:

  Check for a valid BAP suboption length.

packet-mount.c:

  Fix a possible integer overflow in dissect_group().

svn path=/trunk/; revision=7590