Fix bug in LineExtractor.cpp

[RayShark0605]

At line 55 of the original LineExtractor.cpp, if the IF STATEMENT expression here is FALSE, after skipping the IF STATEMENT, the INDEX in the following "_keylines.resize(index + 1);" statement is the length of _KEYLINES, At this time, if it is resized to INDEX+1 (that is, "length plus one"), the attribute in the added data will be a random value, which will lead to the following "lbd->compute(image, _keylines, descriptors); "statement (Calculate the descriptor of the feature line segment) Infinite loop or error!