Fix buffer overflow introduced in 2.73rc6.

Fix buffer overflow introduced in 2.73rc6.

Fix off-by-one in code which checks for over-long domain names
in received DNS packets. This enables buffer overflow attacks
which can certainly crash dnsmasq and may allow for arbitrary
code execution. The problem was introduced in commit b8f16556d,
release 2.73rc6, so has not escaped into any stable release.
Note that the off-by-one was in the label length determination,
so the buffer can be overflowed by as many bytes as there are
labels in the name - ie, many.

Thanks to Ron Bowes, who used lcmatuf's afl-fuzz tool to find
the problem.