streamlookup: fix KeysForLookup reallocation

[yumkam]

It could've been triggered on each read from empty channel

Changelog entry

...

Changelog category

• Not for changelog (changelog entry is not required)

Additional information

...

[github-actions]

⚪ 2024-10-10 11:02:26 UTC Pre-commit check linux-x86_64-relwithdebinfo for 79644c4 has started.
⚪ 2024-10-10 11:02:37 UTC Artifacts will be uploaded here
⚪ 2024-10-10 11:05:28 UTC ya make is running...
🟢 2024-10-10 12:06:28 UTC Tests successful.

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
39703	34129	0	0	5470	104

🟢 2024-10-10 12:09:57 UTC Build successful.
🟢 2024-10-10 12:10:14 UTC ydbd size 2.8 GiB changed* by +2.5 KiB, which is < 100.0 KiB vs main: OK

ydbd size dash	main: f6e0d96	merge: 79644c4	diff	diff %
ydbd size	3 012 083 536 Bytes	3 012 086 104 Bytes	+2.5 KiB	+0.000%
ydbd stripped size	477 604 856 Bytes	477 605 176 Bytes	+320 Bytes	+0.000%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}

[github-actions]

⚪ 2024-10-10 11:04:36 UTC Pre-commit check linux-x86_64-release-asan for 79644c4 has started.
⚪ 2024-10-10 11:04:47 UTC Artifacts will be uploaded here
⚪ 2024-10-10 11:07:43 UTC ya make is running...
🟡 2024-10-10 12:30:04 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
10180	9792	0	64	285	39

🟢 2024-10-10 12:30:57 UTC Build successful.
🟢 2024-10-10 12:31:27 UTC ydbd size 5.7 GiB changed* by +592 Bytes, which is < 100.0 KiB vs main: OK

ydbd size dash	main: f6e0d96	merge: 79644c4	diff	diff %
ydbd size	6 103 959 536 Bytes	6 103 960 128 Bytes	+592 Bytes	+0.000%
ydbd stripped size	1 521 006 832 Bytes	1 521 006 832 Bytes	0 Bytes	0.000%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}