Fix static analyzer warning

nodes.cc: warning: Access to field 'next_' results in a dereference of a
null pointer (loaded from field 'last_instruction_')

This was split from
https://android-review.googlesource.com/#/c/416101/. Please see the
discussion nodes.cc (patch set 1) for why this warning is triggered.

Bug: 32619234
Test: test-art-host. Rebuilt ART with the analyzer to verify that these
issues are gone.

Change-Id: Id5da00ceee0667441233153a7971d238ea8c8650