narrowband SFT naming check broke lalpulsar_splitSFTs for bands < 1 Hz

@karl-wette another small problem from !2027 (merged) : the check in XLALBuildSFTFilenameFromSpec for narrow-band width (here) does not account for non-integer remnants and hence fails when calling lalpulsar_splitSFTs with any band below 1 Hz. This is not caught by the test script because it always uses -fb 8.

As so often the fix is trivial, but I'll better update the test script too to cover both large and small bands, then I'll make an MR.