On overflow of code_gen_buffer, we unlock the guest pages we had been
translating, but failed to clear gen_tb. On restart, if we cannot
allocate a TB, we exit to the main loop to perform the flush of all
TBs as soon as possible. With garbage in gen_tb, we hit an assert:
../src/accel/tcg/tb-maint.c:348:page_unlock__debug: \
assertion failed: (page_is_locked(pd))
Fixes: deba78709a ("accel/tcg: Always lock pages before translation")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
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