Don't fail if no contacts were found

If no crosslinks were satisfied for a pair of
subunits, previously the subplot failed due to
trying to calculate log(0). Set a dummy max
in this case.

pyext/src/analysis.py

@@ -1483,7 +1483,7 @@ def dist_matrix(self, skip_cmap=0, skip_xl=1, outname=None):
                         np.log(mtr),
                         interpolation='nearest',
                         vmin=0.,
-                        vmax=log(mtr.max()))
+                        vmax=log(mtr.max()) if mtr.max() > 0. else 1.)
                     ax.set_xticks([])
                     ax.set_yticks([])
                 if skip_xl == 0: