Mark object as on stack for calling into Ruby
Mark object as on stack for calling into Ruby

When we call back into Ruby here for respond_to_missing, we have to make
sure to mark self as on stack because it might trigger a GC.

Fixes #2445

sferik/rubinius,dblock/rubinius,pH14/rubinius,ngpestelos/rubinius,heftig/rubinius,ngpestelos/rubinius,kachick/rubinius,Azizou/rubinius,dblock/rubinius,ngpestelos/rubinius,jsyeo/rubinius,sferik/rubinius,Wirachmat/rubinius,Azizou/rubinius,sferik/rubinius,kachick/rubinius,Azizou/rubinius,heftig/rubinius,dblock/rubinius,ruipserra/rubinius,Wirachmat/rubinius,Wirachmat/rubinius,heftig/rubinius,kachick/rubinius,sferik/rubinius,dblock/rubinius,benlovell/rubinius,ngpestelos/rubinius,pH14/rubinius,digitalextremist/rubinius,jemc/rubinius,pH14/rubinius,dblock/rubinius,heftig/rubinius,ruipserra/rubinius,ruipserra/rubinius,Wirachmat/rubinius,Azizou/rubinius,benlovell/rubinius,jemc/rubinius,dblock/rubinius,ruipserra/rubinius,ruipserra/rubinius,ruipserra/rubinius,digitalextremist/rubinius,kachick/rubinius,benlovell/rubinius,digitalextremist/rubinius,digitalextremist/rubinius,digitalextremist/rubinius,sferik/rubinius,jemc/rubinius,jsyeo/rubinius,Azizou/rubinius,heftig/rubinius,Wirachmat/rubinius,sferik/rubinius,kachick/rubinius,mlarraz/rubinius,pH14/rubinius,kachick/rubinius,pH14/rubinius,jsyeo/rubinius,mlarraz/rubinius,jemc/rubinius,mlarraz/rubinius,mlarraz/rubinius,mlarraz/rubinius,jemc/rubinius,pH14/rubinius,mlarraz/rubinius,Wirachmat/rubinius,jsyeo/rubinius,kachick/rubinius,jsyeo/rubinius,Azizou/rubinius,jsyeo/rubinius,kachick/rubinius,heftig/rubinius,benlovell/rubinius,dblock/rubinius,digitalextremist/rubinius,ngpestelos/rubinius,ruipserra/rubinius,Azizou/rubinius,jemc/rubinius,sferik/rubinius,jemc/rubinius,mlarraz/rubinius,benlovell/rubinius,heftig/rubinius,benlovell/rubinius,benlovell/rubinius,ngpestelos/rubinius,Wirachmat/rubinius,pH14/rubinius,digitalextremist/rubinius,jsyeo/rubinius,ngpestelos/rubinius