FTP: More EPLF 'm' timestamp parsing fixes

[MegaManSec]

The m case in ftpListParseParts() reversed the strtol() validity check. We broke out when any digits were consumed instead of only when no conversion happened. As a result, valid epoch timestamps were treated as invalid and p->date was often left unset.

Also guard against ctime() returning nil and avoid null pointer dereference if no newline is present in ctime() output.

Also reject m entries with trailing garbage.

[squid-anubis]

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FtpGateway: fix 'm' timestamp parsing; guard ctime(); trim safely (#2207)

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[eduard-bagdasaryan]

run this PR through the tests

Done, no problems were found.

[rousskov]

I adjusted PR description a little to mention trailing garbage handling and to avoid retelling the fairly clear diff when it comes to strstr() refactoring. I am not against adding some text about that refactoring though.