fix: avoid nil dereferences in plugin hooks

[dlh]

Plugin hooks can run before the editor's global state exists:

• onBufPaneOpen fires for the infobar's own pane, which InitGlobals() builds before LogBuf and Tabs are assigned.
• onBufferOpen fires for the user's files, which load before InitTabs().

A handler that calls an action then hits a nil dereference in Go (e.g. buffer.Log() → WriteLog, or micro.CurPane() → MainTab()), and startup stops on "Press enter to continue".

Two commits:

• b706262b: main() and the test helper share one initEditor().
• b7a3882d: nil guards in MainTab(), micro.CurPane(), WriteLog(), SetGlobalOption(), and Tabs.Resize() for layout options set before InitTabs(); the plugin docs now say which API calls return nil during startup.

Example

~/.config/micro/plug/crashdemo/crashdemo.lua:

local buffer = import("micro/buffer")

-- At startup this also fires for the infobar's pane, before the log buffer exists.
function onBufPaneOpen(bp)
    buffer.Log("opened " .. bp.Buf.Path .. "\n")
end

Run micro README.md.

Before fix

Plugin crashdemo: runtime error: invalid memory address or nil pointer dereference
stack traceback:
	[G]: in function 'Log'
	crashdemo:5: in main chunk
	[G]: ?

Press enter to continue

After fix

micro starts normally. The infobar's call is dropped, since the log buffer does not exist yet; the call for README.md reaches the log.