iseq.c (iseq_data_to_ary): add missing GC guard

It seems possible for a compiler to optimize away nbody
because we only access it via RARRAY_AREF and RARRAY_LEN
macros.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48597 b2dd03c8-39d4-4d8f-98ff-823fe69b080e