Don't trust the pointer value in a packet; it could be invalid, and this

Don't trust the pointer value in a packet; it could be invalid, and this
could cause an unsigned length value to be reduced by more than its
value, turning it into a very large value.

I couldn't exactly reproduce bug 6833, but it was due to an attempt to
allocate 4294967110 bytes, and this bug caused remaining_len to equal
4294967110, and it would try to create a reassembled packet tvbuff of
that size, so I'm guessing this fixes 6833.

svn path=/trunk/; revision=41001