Fix overflow in calculating highbd SSIM

Fix overflow in calculating highbd SSIM

Example internal stats
Before the fix:
Bitrate	AVGPsnr	GLBPsnr	AVPsnrP	GLPsnrP	VPXSSIM	VPSSIMP	FASTSIM	PSNRHVS	WstPsnr	WstSsim	WstFast	WstHVS	AVPsnrY	APsnrCb	APsnrCr	  Block	WstBlck	Consist	WstCons	    Time	RcErr	AbsErr
 153.39	 37.131	 36.420	 37.151	 36.437	716.077	817.445	 10.422	 34.347	 32.980	  0.916	  9.281	 30.208	 36.024	 41.830	 40.581	  0.000	  0.000	100.000	100.000	   55006	   2.26	   2.26
No mismatch detected in recon buffers

After the fix:
Bitrate	AVGPsnr	GLBPsnr	AVPsnrP	GLPsnrP	VPXSSIM	VPSSIMP	FASTSIM	PSNRHVS	WstPsnr	WstSsim	WstFast	WstHVS	AVPsnrY	APsnrCb	APsnrCr	  Block	WstBlck	Consist	WstCons	    Time	RcErr	AbsErr
 153.39	 37.131	 36.420	 37.151	 36.437	 69.808	 70.023	 10.422	 34.347	 32.980	  0.910	  9.281	 30.208	 36.024	 41.830	 40.581	  0.000	  0.000	100.000	100.000	   55067	   2.26	   2.26
No mismatch detected in recon buffers

Change-Id: I820abc498c1543548f193874046582b50afd0238