MAINT ignore sklearn/tree/_utils.c in diff
COSMIT tree: unused variable warnings and use for/range

Not recompiled with Cython, as line numbers still match up.
BUG fix unchecked mallocs in trees

All mallocs should be checked now. Some notes for the next person to touch
this code, or do anything with malloc in Cython code:

* with gil: raise MemoryError() didn't work as expected, as a surrounding
  nogil block catches and logs the exception, then proceeds as if nothing
  happened.
* There were several opportunities for double free in the code. This is
  handled by letting the destructor handle the freeing.
* Similarly, constructors failed to set pointers to NULL so __dealloc__
  would try to free unitialized pointers.
* Out of memory might leave trees in an inconsistent state, but will not
  cause a double free. A subsequent successful fit should produce a
  consistent state, although I didn't test this.
* Replaced a free/calloc with a realloc; realloc(NULL, n) = malloc(n).
* free(NULL) is a harmless no-op per ISO C89.

Also performed an optimization: the pre-sort splitter was using size_t for
an array of booleans. unsigned char is faster and smaller.

Finally, fixed some cosmetic issues (long lines).
Merge pull request #2715 from larsmans/tree-malloc

fix malloc errors in trees