linux-user: Conditionally Pass Attribute Pointer to mq_open()

linux-user: Conditionally Pass Attribute Pointer to mq_open()

The mq_open system call takes an optional struct mq_attr pointer
argument in the fourth position.  This pointer is used when O_CREAT
is specified in the flags (second) argument.  It may be NULL, in
which case the queue is created with implementation defined attributes.

Change the code to properly handle the case when NULL is passed in the
arg4 position.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>