Avoidance of undefined-variable use in OBS

Context

The ORCA2_ICE_OBS SETTE test of both NEMO main and branch_4.2 has failed when run in debug mode due to the use of an undefined variable.

Analysis

When ldnightav is .FALSE. in subroutine obs_surf_opt of module obs_oper, the undefined variable idayend is accessed during the evaluation of the condition in two IF constructs.

Fix

The affected IF constructs could be restructured into nested IF constructs that test ldnightav in the first instance.