Fix compilation errors, clang++-15

[ghost]

I've tried to compile the main branch, but there are two minor things which are make CE.
I'm not sure about the second one (num_empty_non_l0_level), probably there is should be additional assert.

-c ../cache/clock_cache.cc
[build] ../cache/clock_cache.cc:855:15: error: variable 'i' set but not used [-Werror,-Wunused-but-set-variable]
[build]   for (size_t i = 0; &array_[current] != h; i++) {
[build]               ^

[build] ../db/version_set.cc:3665:7: error: variable 'num_empty_non_l0_level' set but not used [-Werror,-Wunused-but-set-variable]
[build]   int num_empty_non_l0_level = 0;
[build]       ^
[build] 1 error generated.

[ajkr]

LGTM

[facebook-github-bot]

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