zebra: nhlfe_alloc should not fail

zebra: nhlfe_alloc should not fail

Let's prevent nhlfe_alloc from actually returning anything that can fail:

1) nexthop_new -> never returns NULL so checking for NULL here
makes no sense, remove it.

2) lsp not being NULL is a assert condition here as that it's
a precondition for the function to work properly.

3) since nhlfe_alloc cannot return NULL now remove tests
for it in callng functions

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>