tests: Initialize array in pipe test

As the pointer to the data buffer passed to k_pipe_put() is now a
'const', that buffer must be initialized before calling k_pipe_put()
to avoid a compiler warning.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>

tests/kernel/pipe/pipe_api/src/test_pipe_contexts.c

@@ -353,6 +353,8 @@ ZTEST(pipe_api, test_half_pipe_put_get)
 	size_t rd_byte = 0;
 	int ret;
 
+	memset(rx_data, 0, sizeof(rx_data));
+
 	/* TESTPOINT: min_xfer > bytes_to_read */
 	ret = k_pipe_put(&kpipe, &rx_data[0], 1, &rd_byte, 24, K_NO_WAIT);
 	zassert_true(ret == -EINVAL);