eal: fix lcore accessors for non-EAL threads

eal: fix lcore accessors for non-EAL threads

If rte_lcore_index() is asked to give the index of the
current lcore (argument -1) and is called from a non-EAL thread
then it would invalid result. The result would come
lcore_config[-1].core_index which is some other data in the
per-thread area.

The resolution is to return -1 which is what rte_lcore_index()
returns if handed an invalid lcore.

Same issue existed with rte_lcore_to_cpu_id().

Bugzilla ID: 446
Fixes: 26cc3bbe4dc0 ("eal: add lcore accessors")

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: David Marchand <david.marchand@redhat.com>