x86emul: also put_fpu() on error paths

x86emul: also put_fpu() on error paths

fail_if() and generate_exception_if() could theoretically bypass the
normal flow reaching put_fpu(), and not invoking it would leave the
fpu_exception_callback pointer in place, allowing for the callback to
be called at an unexpected time. Luckily the two
generate_exception_if()-s that would actually trigger this are
currently commented out, so this is not (yet) a (security) issue.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>