share_memory_ needs a check for empty tensor

```
a=torch.randn(0, 100)
a.share_memory_()
```
```
RuntimeError: $ Torch: unable to mmap memory: you tried to mmap 0GB. at /private/home/soumith/pytorch/torch/lib/TH/THAllocator.c:317
```
This error on linux, on OSX it works.