Patch to check the return value from posix_fadvise in fileringbuffer.cpp - fixes coverity 746748 and 766747

[Gary Buhrmaster <gary.buhrmaster@…>]

Static analysis determined that the return value from posix_fadvise was not checked. Check the return, and log if failed (only debug level logging, since fadvise should never fail in the first place, and if it does fail it should not matter (it is, after all, only a hint) but if debugging, the failure might tell one something).

Fixes coverity 746748 and 746747

github ref: ​https://github.com/garybuhrmaster/mythtv/commit/e6bbaf778585fd2b8f930d61164cfe0622744d93

github git-am ref: ​https://github.com/garybuhrmaster/mythtv/commit/e6bbaf778585fd2b8f930d61164cfe0622744d93.patch

[danielk]

If these advisory calls don't work we don't really want to spam the logs. They are just intended to give the operating system a heads up if it supports it.

[Gary Buhrmaster <gary.buhrmaster@…>]

Since the only errors returned per the spec (which would only result in logs if running at the file debug level anyway) will occur with a programming error (bad fd, bad advise option specification), the logs would not be spammed (unless there is something else seriously broken in some new codes). fadvise does not return an error if it simply decides to ignore the offset/sizes (it simple ignores the advise, and moves on), so logs would never be generated in those cases.

[Gary Buhrmaster <gary.buhrmaster@…>]

In 351c36ac73d664a370b9b12c655604c03ba71ca1/mythtv:

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