bytes_count() doesn't check start maximum value: _adjust_indices()
should check that start is smaller than len (smaller or egal? len or
len-1?). Example:

>>> b = bytearray(3)
>>> b.count("x", 1491491034, 0)

start=1491491034 should be replaced by 3 (or 2 or 4? I don't know
bytearray enough).