symbol.c (rb_intern_cstr_without_pindown): check dsymbol on return

This is not a complete fix for bug 10206, but seems to reduce
that crash and also looks correct.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47455 b2dd03c8-39d4-4d8f-98ff-823fe69b080e