Fixes a runtime if there's no mob when a dna injector is injected by adding a sanity check.

git-svn-id: http://tgstation13.googlecode.com/svn/trunk@5432 316c924e-a436-60f5-8080-3fe189b3f50e

code/game/objects/items/weapons/dna_injector.dm

@@ -53,7 +53,8 @@
 				uses--
 
 	spawn(0)//this prevents the collapse of space-time continuum
-		user.drop_from_inventory(src)
+		if (user)
+			user.drop_from_inventory(src)
 		del(src)
 	return uses