bvector<>::{set,clear}_range_no_check: Formally pre-zero tmp_gap_blk.

[ucko]

Reinstate the tmp_gap_blk initializer in set_range_no_check, and add an identical one in clear_range_no_check, to address errors when building with -Werror=maybe-uninitialized.

[ucko]

I reproduced the error, and can confirm that your minimal patch will suffice.

Thanks!