block/file-posix: Fix laio_init() error handling crash bug

block/file-posix: Fix laio_init() error handling crash bug

raw_aio_attach_aio_context() passes uninitialized Error *local_err by
reference to laio_init() via aio_setup_linux_aio().  When laio_init()
fails, it passes it on to error_setg_errno(), tripping error_setv()'s
assertion unless @local_err is null by dumb luck.

Fix by initializing @local_err properly.

Fixes: ed6e2161715c527330f936d44af4c547f25f687e
Cc: Nishanth Aravamudan <[email protected]>
Cc: Stefan Hajnoczi <[email protected]>
Cc: Kevin Wolf <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <20191130194240[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>