pair_peaks: Don't attempt to pair 000

Normally the beam centre is masked out or not on the detector.  However,
when it is, pairing with 000 causes gradients to come out as NaN and the
prediction refinement to fail.

libcrystfel/src/predict-refine.c

@@ -250,6 +250,9 @@ static int pair_peaks(struct image *image, Crystal *cr,
 		k = lrint(kd);
 		l = lrint(ld);
 
+		/* Don't pair with 000, because that can cause trouble later */
+		if ( (h==0) && (k==0) && (l==0) ) continue;
+
 		refl = reflection_new(h, k, l);
 		if ( refl == NULL ) {
 			ERROR("Failed to create reflection\n");