configure: fix malloc check

Avoid random return value.

Fixes commit f2dfe54c

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Message-Id: <20200707171326.16422-1-olaf@aepfle.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>