Cycles: Make MESA compiler more happy

While this compiler is not officially supported yet, getting it to work is
a nice thing because more and more AMD cards will fall under MESA driver.

It's also nice to use explicit comparison with NULL, which makes it more
clear whether variable is a boolean or pointer. Even Rust enforces this!

Patch by Ian Bruce with own modifications.

intern/cycles/kernel/closure/alloc.h

@@ -62,7 +62,7 @@ ccl_device_inline ShaderClosure *bsdf_alloc(ShaderData *sd, int size, float3 wei
 {
 	ShaderClosure *sc = closure_alloc(sd, size, CLOSURE_NONE_ID, weight);
 
-	if(!sc)
+	if(sc == NULL)
 		return NULL;
 
 	float sample_weight = fabsf(average(weight));