bug when to get_total_mfd a big gridded source is passed

[julgp]

When I used "get_total_mfd" to get the Total MDF from a large gridded point source like gridded_seismicity_source_40.xml from SAM model (48107 point sources), I got the following error:
Traceback (most recent call last):
File "./test_01.py", line 87, in
main(sys.argv[1:])
File "./test_01.py", line 83, in main
p.callfunc()
File "/Users/jgarcia/oq/src/oq-engine/openquake/baselib/sap.py", line 181, in callfunc
return self.func(**vars(namespace))
File "./test_01.py", line 66, in make
mdfall1 = get_total_mfd(sources, trt=None)
File "/Users/jgarcia/oq/src/oq-mbtk/openquake/man/checks/mfd.py", line 26, in get_total_mfd
mfdall.stack(mfd)
File "/Users/jgarcia/oq/src/oq-mbtk/openquake/mbt/tools/mfd.py", line 263, in stack
mfd2 = mfd_resample(bin_width, mfd2)
File "/Users/jgarcia/oq/src/oq-mbtk/openquake/mbt/tools/mfd.py", line 375, in mfd_resample
return mfd_downsample(bin_width, mfd)
File "/Users/jgarcia/oq/src/oq-mbtk/openquake/mbt/tools/mfd.py", line 480, in mfd_downsample
return EvenlyDiscretizedMFD(nocc[0, 0], bin_width, list(nocc[:, 3]))
IndexError: index 0 is out of bounds for axis 0 with size 0

With smaller sources the code works (i.e. gridded_seismicity_source_04.xml)
I'm attaching a zip with the sources and test code.
Archive.zip