Fix houghlines.py when no lines found

In the C++ equivalent of this example a check is made whether the vector (here in Python we have a list) actually has any lines in it that is whether the Hough lines function has managed to find any in the given image. This check is missing for the Python example and if no lines are found the application breaks.

samples/python/houghlines.py

@@ -36,16 +36,17 @@ if __name__ == '__main__':
 
     else:    # HoughLines
         lines = cv2.HoughLines(dst, 1, math.pi/180.0, 50, np.array([]), 0, 0)
-        a,b,c = lines.shape
-        for i in range(a):
-            rho = lines[i][0][0]
-            theta = lines[i][0][1]
-            a = math.cos(theta)
-            b = math.sin(theta)
-            x0, y0 = a*rho, b*rho
-            pt1 = ( int(x0+1000*(-b)), int(y0+1000*(a)) )
-            pt2 = ( int(x0-1000*(-b)), int(y0-1000*(a)) )
-            cv2.line(cdst, pt1, pt2, (0, 0, 255), 3, cv2.LINE_AA)
+        if lines != None:
+            a,b,c = lines.shape
+            for i in range(a):
+                rho = lines[i][0][0]
+                theta = lines[i][0][1]
+                a = math.cos(theta)
+                b = math.sin(theta)
+                x0, y0 = a*rho, b*rho
+                pt1 = ( int(x0+1000*(-b)), int(y0+1000*(a)) )
+                pt2 = ( int(x0-1000*(-b)), int(y0-1000*(a)) )
+                cv2.line(cdst, pt1, pt2, (0, 0, 255), 3, cv2.LINE_AA)
 
     cv2.imshow("source", src)
     cv2.imshow("detected lines", cdst)