librdmacm: reject negative indexes in idm_lookup

Negative fd values were not rejected and could index before the idm
array.  Fold the fd >= 0 check into idm_lookup so callers need not
duplicate it.

Signed-off-by: Batsheva Black <bblack@nvidia.com>

librdmacm/indexer.h

diff --git a/librdmacm/indexer.h b/librdmacm/indexer.h
index e6ffc60..c37ea21 100644
--- a/librdmacm/indexer.h
+++ b/librdmacm/indexer.h
@@ -97,7 +97,8 @@
 
 static inline void *idm_lookup(struct index_map *idm, int index)
 {
-	return ((index <= IDX_MAX_INDEX) && idm->array[idx_array_index(index)]) ?
+	return ((index >= 0) && (index <= IDX_MAX_INDEX) &&
+		idm->array[idx_array_index(index)]) ?
 		idm_at(idm, index) : NULL;
 }