[BUG] ebtree: fix ebmb_lookup() with len smaller than the tree's keys

[BUG] ebtree: fix ebmb_lookup() with len smaller than the tree's keys

(from ebtree 6.0.5)

ebmb_lookup() is used by ebst_lookup_len() to lookup a string starting
with a known substring. Since the substring does not necessarily end
with a zero, we must absolutely ensure that the comparison stops at
<len> bytes, otherwise we can end up comparing crap and most often
returning the wrong node in case of multiple matches.

ebim_lookup() was fixed too by resyncing it with ebmb_lookup().
(cherry picked from commit 98eba315aa2c3285181375d312bcb770f058fd2b)

This should be backported to 1.4 though it's not critical there.