Use safe inference in unpacking-non-sequence checker

Unfortunately, this fix means that we won't be able to emit an error for
cases like this one:
```
def foo():
    if True:
        return [1, 2]
    return [3, 4, 5]
a, b = foo()
```
Well, not unless we get flow-sensitive inference. But we still need this fix to
reduce the number of false-positive errors.

Fixes issue #695.

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