iseq.c: avoid segfault on incomplete iseq

Compile failures will trigger iseq_free before
iseq->callinfo_entries are allocated at all.

* iseq.c (iseq_free): avoid segfault on incomplete iseq
* test/ruby/test_syntax.rb (test_invalid_next): new test
  for syntax error, not segfault

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48704 b2dd03c8-39d4-4d8f-98ff-823fe69b080e