Fix for invalid check of SPARSE_VEC_INVALID_INDEX

Fix for invalid check of SPARSE_VEC_INVALID_INDEX

When looking up a UDP port / GRE protocol in the sparse vectors
next_by_dst_port / next_by_protocol a data from the vector was
tested for SPARSE_VEC_INVALID_INDEX instead of sparse index itself.

This doesn’t matter for most cases since V[0] = 0 is true for all
sparse vectors. This however could cause an issue when a valid
sparse entry e.g. V[1234] = 0, with data (0) mistakenly passing
the test for SPARSE_VEC_INVALID_INDEX, while the index itself (1234)
is a valid index.

Change-Id: I04818cc43efeae047a4dae79078157d48b8c359c
Signed-off-by: Alex Popovsky <apopovsk@cisco.com>