efi_loader: SetVariable() fix illegal return

We always have to return via EFI_EXIT() from EFIAPI functions.

Coverity reported an unreachable line and a resource leak.

Fixes: commit 6d2f27c5 ("efi_loader: variable: support APPEND_WRITE")
Reported-by: Coverity Scan CID 253575, CID 184095
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>