mmap.read() crashes when passed a negative count:

    def test_read_negative(self):
        f = open(TESTFN, 'w+')
        f.write("ABCDE\0abcde")
        f.flush()

        mf = mmap.mmap(f.fileno(), 0)

        self.assertEqual(mf.read(2),  "AB")    # OK
        self.assertEqual(mf.read(-1), "CDE")   # ??
        self.assertEqual(mf.read(-1), "BCDE")  # ??
        self.assertEqual(mf.read(-1), "ABCDE") # ??
        mf.read(-1)                            # crash!!

I don't know what mf.read(-1) should do: raise a ValueError, return the empty 
string, or return everything from the current pos to len(mf)?