[MC-154668] Invalid characters crash the game in jigsaw block input upon pressing enter

The bug

Invalid characters crash the game in jigsaw block input upon pressing enter.

How to reproduce

Execute the command:
```
/give @s jigsaw
```
Open the jigsaw block (displaying the user interface)
Type in any of the text boxes, characters: '\' or '{}' (invalid unicode characters blocking the "done" button)
Press the enter key
→ The game crashes

Crash report

1.14.3-pre3: crash-2019-06-16_22.54.27-client.txt

Description: keyPressed event handler

n: Non [a-z0-9/._-] character in path of location: minecraft:empty\\
	at qt.<init>(SourceFile:38)
	at qt.<init>(SourceFile:43)
	at dbo.d(SourceFile:44)
	at dbo.b(SourceFile:35)
	at dbo.keyPressed(SourceFile:106)
	at cvm.a(SourceFile:410)
	at cvm$$Lambda$3043/1332786777.run(Unknown Source)
	at czx.wrapScreenError(SourceFile:441)
	at cvm.a(SourceFile:408)
	at cvm$$Lambda$1477/601932288.invoke(Unknown Source)
	at org.lwjgl.glfw.GLFWKeyCallbackI.callback(GLFWKeyCallbackI.java:37)
	at org.lwjgl.system.JNI.invokeV(Native Method)
	at org.lwjgl.glfw.GLFW.glfwPollEvents(GLFW.java:3101)
	at cuh.l(SourceFile:425)
	at cuh.c(SourceFile:283)
	at cvo.b(SourceFile:1023)
	at cvo.e(SourceFile:976)
	at cvo.b(SourceFile:411)
	at net.minecraft.client.main.Main.main(SourceFile:154)