fix: use fgets return value as loop condition in exec()

[xq9mend]

What

Fix incorrect loop condition in exec() in common/exec.cpp.

Why

Using !feof(pipe) as a loop condition has two problems:

1. feof() only becomes true after a read has already hit EOF, so the loop always executes one extra iteration where fgets() returns NULL.
2. feof() does not detect I/O errors (ferror). On a read error, fgets() returns NULL but feof() remains false, causing an infinite loop.

How

Drive the loop directly on the fgets() return value. This handles both EOF and I/O errors correctly with a single condition, and eliminates the redundant inner if guard.

// Before
while (!feof(pipe))
{
    if (fgets(buffer.data(), buffsz, pipe) != NULL)
    {
        stdout += buffer.data();
    }
}

// After
while (fgets(buffer.data(), buffsz, pipe) != NULL)
{
    stdout += buffer.data();
}

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[xq9mend]

/azpw run Azure.sonic-swss-common

[mssonicbld]

⚠️ Notice: /azpw run only runs failed jobs now. If you want to trigger a whole pipline run, please rebase your branch or close and reopen the PR.
💡 Tip: You can also use /azpw retry to retry failed jobs directly.

Retrying failed(or canceled) jobs...

[mssonicbld]

/azp run

[mssonicbld]

No Azure DevOps builds found for #1211.

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

Hi, there are workflow run(s) waiting for approval, you may be first-time contributor. I will notify maintainers to help approve once PR is approved. Thanks!

_{---Powered by SONiC BuildBot}

[xq9mend]

/azpw run Azure.sonic-swss-common

[mssonicbld]

⚠️ Notice: /azpw run only runs failed jobs now. If you want to trigger a whole pipline run, please rebase your branch or close and reopen the PR.
💡 Tip: You can also use /azpw retry to retry failed jobs directly.

Retrying failed(or canceled) jobs...

[mssonicbld]

Retrying failed(or canceled) stages in build 1144007:

✅Stage Test:

• Job vstest: retried.

[xq9mend]

/azpw run Azure.sonic-swss-common

[mssonicbld]

⚠️ Notice: /azpw run only runs failed jobs now. If you want to trigger a whole pipline run, please rebase your branch or close and reopen the PR.
💡 Tip: You can also use /azpw retry to retry failed jobs directly.

Retrying failed(or canceled) jobs...

[mssonicbld]

Retrying failed(or canceled) stages in build 1144007:

✅Stage Test:

• Job vstest: retried.

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[xq9mend]

The Test vstest failure is a systemic CI issue unrelated to this PR. The p4rt test group in sonic-swss/tests/p4rt/ is failing across all sonic-swss-common PRs (exit code 123, zero tests passing) due to a mismatch between the latest sonic-swss master test code and the pre-built docker-sonic-vs image. This affects every open sonic-swss-common PR right now — see #1211, #1212, #1213, #1214 all showing the same failure.

The code change in this PR is correct and unrelated to p4rt.

[xq9mend]

/azp run Azure.sonic-swss-common

[azure-pipelines]

Commenter does not have sufficient privileges for PR 1211 in repo sonic-net/sonic-swss-common

[xq9mend]

/azp run Azure.sonic-swss-common

[azure-pipelines]

Commenter does not have sufficient privileges for PR 1211 in repo sonic-net/sonic-swss-common