Bug 710361 - GError corruption in imapx_command_fetch_message_done()

camel_imapx_job_take_error() was being called twice on the same GError.
The 2nd call freed the GError set in the 1st call, which corrupted the
memory.  Put a safeguard in camel_imapx_job_take_error() to catch this
in the future.