fixes index out of range error in polygonization edge tracing setup

pysnic/algorithms/polygonize.py

@@ -343,6 +343,7 @@ def trace_edges_unsafe(ex, ey, direction, segmentation):
     queue = [starting_idx]
 
     while len(queue) != 0:
+        do_break = False
         while True:
             # get next corner and check remaining directions
             idx = queue[-1]
@@ -351,9 +352,15 @@ def trace_edges_unsafe(ex, ey, direction, segmentation):
             l_dirs = len(directions)
             if l_dirs <= 1:
                 idx = queue.pop()
-            if l_dirs != 0:
+                if len(queue) == 0:
+                    do_break = True
+                    break
+            else:
                 direction = directions.pop()
                 break
+
+        if do_break:
+            break
 
         # compute starting coordinates
         px = (idx % widthp) - 1