integrate_rings_once: Handle case where reflection is negative by < 1 photon

libcrystfel/src/integration.c

@@ -1487,6 +1487,11 @@ static int integrate_rings_once(Reflection *refl, struct image *image,
 	 * appropriate size */
 	if ( intensity < -aduph ) {
 		sig2_poisson = -aduph*intensity;
+	} else if ( intensity < 0.0 ) {
+		/* If the intensity is negative (by less than one
+		 * photon), assume the reflection is very weak and
+		 * therefore has a Poisson error of one photon. */
+		sig2_poisson = aduph;
 	}
 
 	sigma = sqrt(sig2_poisson + bx->m*sig2_bg);