accel/tcg: Clear tcg_ctx->gen_tb on buffer overflow

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: deba787 ("accel/tcg: Always lock pages before translation")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

accel/tcg/translate-all.c

@@ -374,6 +374,7 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
                           "Restarting code generation for "
                           "code_gen_buffer overflow\n");
             tb_unlock_pages(tb);
+            tcg_ctx->gen_tb = NULL;
             goto buffer_overflow;
 
         case -2: