Bug fix #2431

Underflow case in Blinn spec... resulting in large negative values to be
returned in certain cases, causing (again!) black dots.
Thanks randall for clear report with nice demo file. :)

source/blender/render/intern/source/rendercore.c

@@ -794,6 +794,7 @@ float Blinn_Spec(float *n, float *l, float *v, float refrac, float spec_power )
 	ang = saacos(nh);
 
 	i= f * g * exp((double)(-(ang*ang) / (2.0*spec_power*spec_power)));
+	if(i<0.0) i= 0.0;
 	
 	return i;
 }