tools:iio: save errno first

The man-page of errno states, that errno should be saved before doing any
library call, as that call may have changed the value of errno. So, when
encountering any error, save errno first.
This patch affects generic_buffer.c, iio_event_monitor.c and iio_utils.c.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>