Fix segfault when passing a long string to v.drawString

If a long string is passed to v.drawString, the game would segfault due to the string copy mechanism not resizing it's string buffer to a large enough size. This can be reproduced with this Lua script:

hud.add(function (v, stplyr, cam)
	local s = "aaaaa"
	for i = 0,5 do
		s = s .. s
	end
	v.drawString(0, 0, s)
end)

The fix simply makes sure to keep expanding the buffer size until it's ensured that there is enough space to fit the new string.