Fixes: runtime error: signed integer overflow: -9166684017437101870 + -2495066639299164439 cannot be represented in type

Fixes: Chromium bug 791349
Reported-by: Matt Wolenetz <wolenetz@google.com>
Reviewed-by: Matt Wolenetz <wolenetz@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>