Allow reshaping blobs to size 0.

[erictzeng]

The current implementation of Blob::Reshape has a check to ensure that Blob sizes do not exceed INT_MAX that goes awry whenever the size of the Blob is 0. (More precisely, the check causes a floating point exception if a 0 appears in the shape anywhere except for the last element.) This PR disables that check if count_ == 0.

While this seems a little silly upon first glance, I think there is a very real use case for supporting this behavior. I'm training a detection network that uses ground truth boxes, which are typically represented by 5 values, but some images have no ground truth boxes at all. In this case, having a ground truth blob of shape (0, 5) seems perfectly sensible to me.

This PR also includes a simple test that reshapes a Blob to shape (0, 5), which would have failed prior to the fix.

[jeffdonahue]

Thanks @erictzeng, LGTM (can merge once Travis passes). The current behavior (allowing empty blobs only in the case that 0 is the last (and only the last) dimension, and otherwise failing with a floating point exception) is clearly wrong. An alternative to this which would also at least make behavior consistent would be to die on all empty blobs, but I don't see a compelling reason to go that way.