af_lavrresample: don't call swr_set_compensation() unless necessary

This was _always_ called, even if the resampling was static, or the
filter was inserted for format conversion only. This should have been
fine, as I expected the function not to enable resampling when the
compensation is unset, and the source/target rates are the same. But
this is not the case, and it always enables resampling.

So explicitly avoid the call. If we have already called it successfully,
it's better not do avoid it (to overwrite the previous compensation
value), but it will also be cheap/no-op then.

Probably fixes #4716.

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