tests: remove warnings on Centos 7 in iio_writedev/iio_readdev

We were getting:
error: 'name' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     printf("Example : " MY_NAME " -u %s -b 256 -s 1024 %s\n",

because, well - we were using it uninitialized...

Initialize it properly

Signed-off-by: Robin Getz <robin.getz@analog.com>

tests/iio_readdev.c

@@ -276,7 +276,7 @@ int main(int argc, char **argv)
 		unsigned int nb_devices = iio_context_get_devices_count(ctx);
 
 		for (i = 0; i < nb_devices; i++) {
-			const char *dev_id, *label, *name;
+			const char *dev_id = NULL, *label = NULL, *name = NULL;
 			bool hit;
 
 			dev = iio_context_get_device(ctx, i);

tests/iio_writedev.c

@@ -289,7 +289,7 @@ int main(int argc, char **argv)
 		unsigned int nb_devices = iio_context_get_devices_count(ctx);
 
 		for (i = 0; i < nb_devices; i++) {
-			const char *dev_id, *label, *name;
+			const char *dev_id = NULL, *label = NULL, *name = NULL;
 			bool hit;
 
 			dev = iio_context_get_device(ctx, i);