adb: fix progress percentage when pulling symlinks.

The adb protocol currently only supports lstat with no way of finding
the target of a symlink, so pulling a symlink that points to a file
looks like pulling a file with length equal to the length of path to
the symlink's target. Pulling a file that's sufficiently large can
overflow the int used to calculate percentage, and result in a bogus
completion percentage being displayed.

Bug: http://b/29277448
Test: adb pull /dev/block/platform/soc.0/f9824900.sdhci/by-name/system
Change-Id: I42d180550ac2aa9e4705676ccbb20f5db789fb8d