Retrieve string data after we stop threads that could GC
Retrieve string data after we stop threads that could GC

This could read invalid data in c_str because that string buffer was
retrieved before handling auxilary threads. But that handling could
result in a GC which would move the pointer that c_str would point at.

By retrieving the pointer after GC'ing we make sure we have the correct
data. Also improve the error message if we fail to run `` so we have
more useful information.

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