Same fix as with the houghcircles

Moved second `imshow()` inside the if-statement to prevent error when no lines have been found and the function is called with an empty `cdst`.

samples/python/houghlines.py

@@ -47,7 +47,8 @@ if __name__ == '__main__':
                 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("detected lines", cdst)
 
     cv2.imshow("source", src)
-    cv2.imshow("detected lines", cdst)
     cv2.waitKey(0)