core: Truncate socket host by null bytes

[Dinnerbone]

Example:

socket.connect("127.0.0.1" + unescape("%00") + "this part is ignored, flash stops at the null byte")

Flash truncates at any null byte - likely a low level method thinking it's getting a cstring. This emulates the truncation behaviour, fixing a Habbo connection issue from Discord.

No tests because our test suite won't run into this, it ignores hosts and everything is local.

[Dinnerbone]

Added unit test!