select: Prevent negative index lookup on osmo_fd_lookup.table

Marked as WIP because I'm not entirely sure if negative index lookups
were allowed on purpose (although I doubt it)

Change-Id: Iba81873cbc435229599b1a8139cf4b627f8e6924
---
M src/core/select.c
1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/src/core/select.c b/src/core/select.c
index 69fa763..70047f0 100644
--- a/src/core/select.c
+++ b/src/core/select.c
@@ -522,7 +522,7 @@
  *  \returns \ref osmo_fd for \ref fd; NULL in case it doesn't exist */
 struct osmo_fd *osmo_fd_get_by_fd(int fd)
 {
-	if (fd > maxfd)
+	if (fd > maxfd || fd < 0)
 		return NULL;
 	return osmo_fd_lookup.table[fd];
 }

