strrb: Remove a tautology from the code

This already came up during review but now that Coverity complains
about it as well, let us just remove it. The variable is unsigned
so it can never be < 0.

Fixes: Coverity CID 1040669.
diff --git a/src/strrb.c b/src/strrb.c
index b137acb..a36205a 100644
--- a/src/strrb.c
+++ b/src/strrb.c
@@ -118,7 +118,7 @@
 {
 	if (osmo_strrb_is_empty(rb))
 		return 0;
-	if ((bufi >= rb->size) || (bufi < 0))
+	if (bufi >= rb->size)
 		return 0;
 	if (rb->start < rb->end)
 		return (bufi >= rb->start) && (bufi < rb->end);