parsers: fix invariant bug in find_deepest (issue5623) find_deepest is used to find the "best" ancestors given a list. In the main loop it keeps an invariant called 'ninteresting' which is supposed to contain the number of non-zero entries in the 'interesting' array. This invariant is incorrectly maintained, however, which leads the the algorithm returning an empty result for certain graphs. This has been fixed. Also, the 'interesting' array is supposed to fit 2^ancestors values, but is incorrectly allocated to twice that size. This has been fixed as well. The tests in test-ancestor.py compare the Python and C versions of the code, and report the error correctly, since the Python version works correct. Even so, I have added an additional test against the expected result, in the event that both algorithms have an identical error in the future. This fixes issue5623.