fwhm min/max: produce results for N_FWHM < 4

[dstndstn]

This small fix allows the code to work with very small catalogs. I encountered this in one of my unit test cases that uses tiny images -- 8 sources in this case, with Nfwhm here being 3.

In my case, this ended up computing fwhmmin = 2 and fwhmmax = 0, so all stars got eliminated by the FWHM check.

In the code, it looks like you planned to handle this case by forcing nw to be at least 1, but then in the for loop the nw is recomputed without that check.