white phosphorus smoke can no longer override stronger flames (#6137)

# About the pull request
white phosphorus smoke will no longer forcefully set the reagent of the
mob

# Explain why it's good for the game
when hit by a WP rocket, a mob is supposed to be set with the blueflame
reagent but sometimes the phosphorus smoke can override it just as you
leave the fire and the mob's fire reagent ends up being normal UT
napalm, which is signficantly weaker than blueflame.

# Testing Photographs and Procedure
yes I tested it

# Changelog
:cl:
fix: White phosphorus will no longer forcefully set a mob's fire_reagent
to be UT napthal
/:cl:

code/game/objects/effects/effect_system/smoke.dm

@@ -240,9 +240,9 @@
 	if(isyautja(M) || isxeno(M))
 		burn_damage *= xeno_yautja_reduction
 
+	var/reagent = new /datum/reagent/napalm/ut()
 	M.burn_skin(burn_damage)
-	M.adjust_fire_stacks(applied_fire_stacks)
-	M.fire_reagent = new /datum/reagent/napalm/ut()
+	M.adjust_fire_stacks(applied_fire_stacks, reagent)
 	M.IgniteMob()
 	M.updatehealth()