vppinfra: fix sparse_vec_index() returning non-zero index for some nonexistent elements

vppinfra: fix sparse_vec_index() returning non-zero index for some nonexistent elements

If we are testing for an index whose membership bitmask happens to sit in LSB of uword,
and there is nothing else set in that uword in the member bitmask, the shortcut path
returning the valid index is taken even if the element with a given index doesn't
exist in the sparse vector. This happens because the count of leading zeroes on the
value of 0 is zero, which is equal to the value taken modulo bitsize uword.

Take care of that case.

Also add unittests showing the problem and verifying that sparse_vec_index2 does not
have the same issue.

Change-Id: I19117e13817c3e5de579b9250bb741de42491985
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>