lib: Link State memory corruption

lib: Link State memory corruption

In function ls_find_subnet(), prefix argument is directly copied into
subnet.key structure to find corresponding subnet in RB Tree. This could leadr
to a memory corruption. Function prefix_copy() must be used instead.

This patch replaces the direct prefix copy by a call to prefix_copy() function
to avoid this memory issue.

Signed-off-by: Olivier Dugeon <olivier.dugeon@orange.com>