tcg: Reset data_gen_ptr correctly

tcg: Reset data_gen_ptr correctly

This pointer needs to be reset after overflow just like
code_buf and code_ptr.

Cc: qemu-stable@nongnu.org
Fixes: 57a269469db ("tcg: Infrastructure for managing constant pools")
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit a7cfd751fb269de4a93bf1658cb13911c7ac77cc)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>