Move DCHECK into loop.

Rationale:
Since pointer is non-null on loop entry, moving the check
into the while loop (rather than after) gives a more friendly
error message in debug mode when something goes wrong.

BUG=27825731
BUG=27627004

Change-Id: I2ea67b3ab6c4edc6815fea38a436e5c3c194e540
1 file changed