check self.uploadProgress inside of dispatch_async

Check self.uploadPorgess is nil or not inside dispatch_async block.
If self.uploadProgress is checked outside of dispatch_async block,
thread can not guaranteed self.uploadPorgress is not nil.

Signed-off-by: Kyungkoo Kang <kyungkoo.kang@yahoo.com>
Merge pull request #2088 from kyungkoo/master

check self.uploadProgress inside of dispatch_async