wayland-server: Properly handle EAGAIN from wl_connection_read()

commit 3cddb3c6 casted len to an
unsigned value to compare to sizeof results.  However,
wl_connection_read() can fail, setting errno to EAGAIN and returning
a value of -1.

When cast to an unsigned type this leads to a loop condition of true
when it should be false.

Signed-off-by: Dipen Somani <dipen.somani@samsung.com>
Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>