elf: Fix program::lookup

Found the problem while running tst-resolve.so, follow the output:
Success: nonexistant = 0
Failed: debug = -443987883
Failed: condvar_wait = -443987883
The time: 1392070630
2 failures.

Bisect pointed to the commit 1dc81fe5483.
After understanding the actual purpose of the changes introduced by this
commit, I figured out that program::lookup simply lacks a return when the
target symbol is found from the underlying module.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>