target/m68k: fix TCG variable double free

t64 is also unconditionally freed after the switch () { ... }

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20180119114444.7590-1-laurent@vivier.eu>
