fix sqrt invalid value warning in footprint correction

eos/projection.py

@@ -224,6 +224,7 @@ class LZProjection(ProjectionInterface):
         # do not perform gravity correction for footprint, would require norm detector distance that is unknown here
         fp_corr_lz = np.where(np.absolute(delta_lz+norm.angle)>5e-3,
                               (delta_lz+self.angle)/(delta_lz+norm.angle), np.nan)
+        fp_corr_lz[fp_corr_lz<0] = np.nan
         self.data.mask &=  np.logical_not(np.isnan(fp_corr_lz))
         self.data.norm = norm_lz*fp_corr_lz
         self.norm_monitor = norm.monitor