tools: iio: iio_generic_buffer: initialize channel array pointer

Uninitialized channel pointer causes segmentation fault when we
call free(channel) during cleanup() with no channels initialized.
This happens when you exit early for usage errors.  Initialize
the pointer to NULL when it is declared.
Signed-off-by: NAlison Schofield <amsfield22@gmail.com>
Cc: Daniel Baluta <daniel.baluta@gmail.com>
Tested-by: NGregor Boirie <gregor.boirie@parrot.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

tools/iio/iio_generic_buffer.c

@@ -348,7 +348,7 @@ int main(int argc, char **argv)
 	int notrigger = 0;
 	char *dummy;
 
-	struct iio_channel_info *channels;
+	struct iio_channel_info *channels = NULL;
 
 	register_cleanup();