[MC-121282] Server icons erased on loading multiplayer menu

As the title says: the game thinks there's an invalid character:

Invalid icon for server <name> (<ip>)
java.lang.IllegalArgumentException: Illegal base64 character a
    at java.util.Base64$Decoder.decode0(Base64.java:714)
    at java.util.Base64$Decoder.decode(Base64.java:623)
    at boz.g(SourceFile:211)
    at boz.a(SourceFile:143)
    at bkv.a(SourceFile:33)
    at bla.a(SourceFile:413)
    at bla.a(SourceFile:211)
    at bow.a(SourceFile:310)
    at bwe.a(SourceFile:1004)
    at bjh.b(SourceFile:853)
    at bjh.a(SourceFile:371)
    at net.minecraft.client.main.Main.main(SourceFile:134)

Invalid icon for server <name> (<ip>)
java.lang.IllegalArgumentException: Input byte array has incorrect ending byte at 392
	at java.util.Base64$Decoder.decode0(Base64.java:742)
	at java.util.Base64$Decoder.decode(Base64.java:623)
	at boz.g(SourceFile:211)
	at boz.a(SourceFile:143)
	at bkv.a(SourceFile:33)
	at bla.a(SourceFile:413)
	at bla.a(SourceFile:211)
	at bow.a(SourceFile:310)
	at bwe.a(SourceFile:1004)
	at bjh.b(SourceFile:853)
	at bjh.a(SourceFile:371)
	at net.minecraft.client.main.Main.main(SourceFile:134)