libssh2: tidy up a knownhost function

[bagder]

• replace atoi() with curlx_str_number()
• use array instead of pointers
• narrow variable scopes

[testclutch]

Analysis of PR #19479 at ad41c762:

Test 2405 failed, which has NOT been flaky recently, so there could be a real issue in this PR. Note that this test has failed in 10 different CI jobs (the link just goes to one of them).

Test 1904 failed, which has NOT been flaky recently, so there could be a real issue in this PR. Note that this test has failed in 12 different CI jobs (the link just goes to one of them).

Test 853 failed, which has NOT been flaky recently, so there could be a real issue in this PR. Note that this test has failed in 12 different CI jobs (the link just goes to one of them).

Generated by Testclutch