internal math of the fastrand function uses a multiply
and add that overflows a signed int.  This triggers a
ubsan failure:

../../unit_test/../unit_test/unit_test.h:60:33: runtime error: signed integer overflow: 56248274 * 214013 cannot be represented in type 'int'

This change casts the intermediate math to unsigned
int to avoid the overflow.

For more info on ubsan, see
http://dev.chromium.org/developers/testing/undefinedbehaviorsanitizer

TESTED=Passing compilation using:
GYP_DEFINES="ubsan=1"
GYP_DEFINES="ubsan_vptr=1"

R=harryjin@google.com, pbos@webrtc.org
BUG=libyuv:563

Review URL: https://codereview.chromium.org/1662453003 .