push: fix segfault for odd config

push: fix segfault for odd config

If you have a branch.$X.merge config option, but no branch.$X.remote, and
your configuration tries to push tracking branches, git will segfault.

The problem is that even though branch->merge_nr is 1, you don't actually
have an upstream since there is no remote.  Other callsites generally
check explicitly that branch->merge is not NULL, so let's do that here,
too.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>