All Reproject functions output NaN

Hi,

I have been trying to use reproject to project PACS160, SPIRE250 and LABOCA images to SPIRE350. Reproject works fine for SPIRE250 to SPIRE350, but gives out NaNs for the rest. Additionally, It worked fine for me few weeks back, but now, with the same commands, it doesn't seem to. The commands are as follows.

from reproject import reproject_exact
hdu1 = fits.open('g326_350_cut.fits')
hdu2 = fits.open('g326_160_convolved.fits')
result1,result2 = reproject_exact((hdu2[0].data,hdu2[0].header),hdu1[0].header)
fits.writeto('g326_160_reproj.fits', result1, hdu1[0].header, overwrite=True)

The output fits file has NaNs and result2 contains zeros. I have also attached a pair of fits files. @astrofrog
[dataset.zip](https://github.com/astropy/reproject/files/6624075/dataset.zip)

The report calls reproject_exact with FITS HDU data and headers and includes dataset.zip; first reproduce the NaNs using the commands shown, comparing the failing PACS160 and LABOCA cases with SPIRE250. Trace reproject_exact from its public entry point to determine why result1 becomes NaN and result2 is zero. Done means the failing case is understood, covered by a regression test, and produces the expected finite output.

Written by the indexing model from the issue text.