[MC-122473] Game crashes when resourcepack selected (path of location: minecraft:blocks/stone frame)

The bug

Game crashes when resource pack selected.

How to reproduce

Start Minecraft 1.12.x and load a resource pack
Close Minecraft and start latest snapshot with resource pack still selected
→ Game crashes while launching

Crash report

17w47b: crash-2017-11-25_10.58.20-client.txt

Description: Initializing game

java.lang.IllegalArgumentException: Non [a-z0-9/._-] character in path of location: minecraft:blocks/stone frame
	at mp.<init>(SourceFile:31)
	at mp.<init>(SourceFile:36)
	at cju.a(SourceFile:688)
	at cju.k(SourceFile:562)
	at cju.n(SourceFile:699)
	at cju.a(SourceFile:188)
	at cjv.a(SourceFile:23)
	at sc.a(SourceFile:126)
	at bmh.an(SourceFile:490)
	at bmh.a(SourceFile:361)
	at net.minecraft.client.main.Main.main(SourceFile:134)