in_tail: tweak the return code of tail_scan_pattern()
in_tail: tweak the return code of tail_scan_pattern()

Currently tail_scan_pattern() returns -1 if no file is found to
match the given pattern. For this reason, the caller was unable
to distinguish "no file found" and "an unexpected error occured".

Make it sure to return 0 if no file is found. This should allow
flb_tail_scan() to emit this message:

    [debug] 0 new files found on path 'C:\*\test.log'

instead of:

    [warn] error scanning path: C:\*\test.log

Signed-off-by: Fujimoto Seiji <ef879192bb1e41d519c82cf76440dc8e9a530806@ceptord.net>

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