pppdump: Avoid out-of-range access to packet buffer

pppdump: Avoid out-of-range access to packet buffer

This fixes a potential vulnerability where data is written to spkt.buf
and rpkt.buf without a check on the array index.  To fix this, we
check the array index (pkt->cnt) before storing the byte or
incrementing the count.  This also means we no longer have a potential
signed integer overflow on the increment of pkt->cnt.

Fortunately, pppdump is not used in the normal process of setting up a
PPP connection, is not installed setuid-root, and is not invoked
automatically in any scenario that I am aware of.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>