Witness::GoalTest is incorrect

[Samarium150]

@arc-s14 Witness::GoalTest method contains bugs

11e64fc:

daff8ca:

Some obvious ones:

• If node.path is empty, then calling node.path.back() will cause undefined behaviour (memory access out of bound).

  hog2/environments/Witness.h

  Lines 2859 to 2861 in e470a47

  	if (node.path.back().second <= height && node.path.back().first <= width && node.path.back().second >= 0 &&
  	node.path.back().first >= 0)
  	verdict = false; //return false;

• Variables should be initialized.

  hog2/environments/Witness.h

  Lines 2908 to 2909 in e470a47

  	int x_c;
  	int y_c;