SNRPDF: silence math domain error

- when initializing an SNR PDF cache with new entries, a math domain error
  can occur when an instrument combination is impossible.  this patch fixes
  it
- it's very odd that this code worked in the past.  Python must be changing
  its behaviour.  commit 29a7482f contains
  both the original version of this code and the data file it successfully
  generated at the time ... so ... it must have worked.  I don't know what
  else to say, but I'm not crazy.

gstlal-inspiral/python/stats/inspiral_extrinsics.py

@@ -423,7 +423,7 @@ class SNRPDF(object):
 				with numpy.errstate(divide = "ignore"):
 					lnbinnedarray.array = numpy.log(lnbinnedarray.array)
 				pdf = rate.InterpBinnedArray(lnbinnedarray, fill_value = NegInf)
-				self.snr_joint_pdf_cache[key] = self.cacheentry(math.log(P_instruments[instruments]), pdf, binnedarray)
+				self.snr_joint_pdf_cache[key] = self.cacheentry(math.log(P_instruments[instruments]) if P_instruments[instruments] else NegInf, pdf, binnedarray)
 
 
 	@staticmethod