Fix parent directory creation for absolute POSIX paths (#9777)

The `[^/]+/` match pattern requires at least on non-slash character
before the trailing slash. This works on Windows where absolute paths
start with a drive letter.

With absolute POSIX paths, the leading slash is skipped by this pattern
causing the accumulated `dirStr` to not start with a slash. `dirStr` is
then interpreted as a relative path, causing the directories to be
erroneously created in the current working directory.

This change fixes the erroneous behavior with absolute POSIX paths by
initializing `dirStr` with a slash if the `fullPath` starts with a
slash. It retains the current behavior on Windows systems.

Author: meehl

src/UpdateCheck.lua

@@ -305,7 +305,7 @@ local ops = { }
 local opsRuntime = { }
 for _, data in pairs(updateFiles) do
 	-- Ensure that the destination path of this file exists
-	local dirStr = ""
+	local dirStr = data.fullPath:sub(1,1) == "/" and "/" or ""
 	for dir in data.fullPath:gmatch("([^/]+/)") do
 		dirStr = dirStr .. dir
 		MakeDir(dirStr)