accel/tcg: Fix undefined shift in store_whole_le16

The computation is documented as unused in this case,
but triggers an ubsan error:

../accel/tcg/ldst_atomicity.c.inc:837:33: runtime error: shift exponent -32 is negative
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../accel/tcg/ldst_atomicity.c.inc:837:33 in

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230606171629.98157-1-richard.henderson@linaro.org>

accel/tcg/ldst_atomicity.c.inc

@@ -833,7 +833,9 @@ static uint64_t store_whole_le16(void *pv, int size, Int128 val_le)
     }
     store_atom_insert_al16(pv - o, v, m);
 
-    /* Unused if sz <= 64. */
+    if (sz <= 64) {
+        return 0;
+    }
     return int128_gethi(val_le) >> (sz - 64);
 }