use mix to blend, avoid division by 0

src/Shaders/depthOfFieldMerge.fragment.fx

@@ -11,5 +11,5 @@ void main(void)
     vec4 blurred = texture2D(textureSampler, vUV);
     vec4 original = texture2D(originalSampler, vUV);
     float coc = texture2D(circleOfConfusionSampler, vUV).r;
-    gl_FragColor = (original * (1.0-coc))+(blurred*coc);
+    gl_FragColor = mix(original, blurred, coc);
 }

src/Shaders/kernelBlur.fragment.fx

@@ -65,6 +65,10 @@ void main(void)
 	#endif
 
 	#ifdef DOF
+		// If there are no samples to blend, make pixel black.
+		if(sumOfWeights == 0.0){
+			gl_FragColor = vec4(0.0,0.0,0.0,1.0);
+		}
 		gl_FragColor /= sumOfWeights;
 	#endif
 }