mptcp: Fix bit-magic to find free index

Actually, ffs() uses integers to find for the next free index. Thus, it
fills the remaining bits of the 8-bit bitfield by zero's. So, we won't
find the correct index.

This patch adds a check, to see if we are going out-of-bounds.

Reported-by: Vinothkumar Vivekanandan <vvinoth@watchy.in>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>