fix for ubsan on unittest.h fastrand()

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 .

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 1570
+Version: 1571
 License: BSD
 License File: LICENSE
 

include/libyuv/version.h

@@ -11,6 +11,6 @@
 #ifndef INCLUDE_LIBYUV_VERSION_H_  // NOLINT
 #define INCLUDE_LIBYUV_VERSION_H_
 
-#define LIBYUV_VERSION 1570
+#define LIBYUV_VERSION 1571
 
 #endif  // INCLUDE_LIBYUV_VERSION_H_  NOLINT

unit_test/unit_test.cc

@@ -20,7 +20,7 @@
 // TODO(fbarchard): Add command line parsing to pass this as option.
 #define BENCHMARK_ITERATIONS 1
 
-int fastrand_seed = 0xfb;
+unsigned int fastrand_seed = 0xfb;
 
 DEFINE_int32(libyuv_width, 0, "width of test image.");
 DEFINE_int32(libyuv_height, 0, "height of test image.");

unit_test/unit_test.h

@@ -55,10 +55,10 @@ static inline double get_time() {
 }
 #endif
 
-extern int fastrand_seed;
+extern unsigned int fastrand_seed;
 inline int fastrand() {
-  fastrand_seed = fastrand_seed * 214013 + 2531011;
-  return (fastrand_seed >> 16) & 0xffff;
+  fastrand_seed = fastrand_seed * 214013u + 2531011u;
+  return static_cast<int>((fastrand_seed >> 16) & 0xffff);
 }
 
 static inline void MemRandomize(uint8* dst, int64 len) {