adb: don't divide by zero

If we stat a file and get a size of 0, and then successfully read bytes
from that file, we would previously divide by zero when calculating the
percentage completion of the file. This case happens either when we're
racing against something else writing to the file, or when we're pulling
magical files such as the ones in /dev/cpuctl/ that lie about their
size.

Bug: http://b/25925733
Change-Id: I980b9c14f44a1eb4a42bc8736c94fa6db06c08d1