[MC-30449] Looting bow server crash

If an entity is killed with a bow that has a negative looting enchantment in multiplayer, the server will crash. In single player, the game will crash.

A negative looting bow can be obtained with the methods used in ~~MC-18301~~ and ~~MC-26104~~, and according to the resolution to ~~MC-18301~~, being able to obtain it is "intended".

Stack trace from server console:

 [SEVERE] Encountered an unexpected exception u
u: Ticking entity
	at net.minecraft.server.MinecraftServer.t(SourceFile:474)
	at ir.t(SourceFile:262)
	at net.minecraft.server.MinecraftServer.s(SourceFile:405)
	at net.minecraft.server.MinecraftServer.run(SourceFile:339)
	at hh.run(SourceFile:582)
Caused by: java.lang.IllegalArgumentException: n must be positive
	at java.util.Random.nextInt(Random.java:250)
	at rp.b(SourceFile:107)
	at oe.a(SourceFile:651)
	at oe.a(SourceFile:612)
	at ro.a(SourceFile:128)
	at ug.l_(SourceFile:265)
	at abv.a(SourceFile:1349)
	at jr.a(SourceFile:479)
	at abv.g(SourceFile:1327)
	at abv.h(SourceFile:1234)
	at jr.h(SourceFile:364)
	at net.minecraft.server.MinecraftServer.t(SourceFile:470)
	... 4 more