No SegFault when unwrapping a 3D fully masked array

The unwrap_phase function no longer causes a Segmentation fault when passed
a 3D array in which all elements are masked.

Closes #1347

skimage/restoration/tests/test_unwrap.py

@@ -186,5 +186,22 @@ def test_unwrap_2d_all_masked():
     assert unwrap[0, 0] == 0
 
 
+def test_unwrap_3d_all_masked():
+    # all elements masked
+    image = np.ma.zeros((10, 10, 10))
+    image[:] = np.ma.masked
+    unwrap = unwrap_phase(image)
+    assert np.ma.isMaskedArray(unwrap)
+    assert np.all(unwrap.mask)
+
+    # 1 unmasked element, still zero edges
+    image = np.ma.zeros((10, 10, 10))
+    image[:] = np.ma.masked
+    image[0, 0, 0] = 0
+    unwrap = unwrap_phase(image)
+    assert np.ma.isMaskedArray(unwrap)
+    assert np.sum(unwrap.mask) == 999   # all but one masked
+    assert unwrap[0, 0, 0] == 0
+
 if __name__ == "__main__":
     run_module_suite()

skimage/restoration/unwrap_3d_ljmu.c

@@ -1136,9 +1136,11 @@ void unwrap3D(double *wrapped_volume, double *unwrapped_volume,
                 &params);
   normalEDGEs(voxel, edge, volume_width, volume_height, volume_depth, &params);
 
-  // sort the EDGEs depending on their reiability. The VOXELs with higher
-  // relibility (small value) first
-  quicker_sort(edge, edge + params.no_of_edges - 1);
+  if (params.no_of_edges != 0) {
+      // sort the EDGEs depending on their reiability. The VOXELs with higher
+      // relibility (small value) first
+      quicker_sort(edge, edge + params.no_of_edges - 1);
+  }
 
   // gather VOXELs into groups
   gatherVOXELs(edge, &params);