Fix log._eval_is_rational: check for zero

This fixes travis failure in the master:

$ ./bin/test sympy/integrals/tests/test_manual.py --rerun 10
===================================================== test process starts ======================================================executable:         /usr/bin/python  (2.7.3-final-0) [CPython]
architecture:       32-bit
cache:              yes
ground types:       gmpy 1.15
random seed:        41680150
hash randomization: on (PYTHONHASHSEED=3487214350)

sympy/integrals/tests/test_manual.py[17] ..............F..                                                                [FAIL]
_____________________________________________________________________________________________________________________________________________________________________ sympy/integrals/tests/test_manual.py:test_issue_6746 _____________________________________  File "/home/sk/src/sympy/sympy/sympy/integrals/tests/test_manual.py", line 226, in test_issue_6746
    assert manualintegrate((y + 1)**x, x) == x
AssertionError

==================================== tests finished: 16 passed, 1 failed, in 189.46 seconds ====================================DO *NOT* COMMIT!