tests/plugin: prevent uninitialized warning

tests/plugin: prevent uninitialized warning

According to the glibc function requirements, we need initialise
 the variable. Otherwise there will be compilation warnings:

glib-autocleanups.h:28:3: warning: ‘out’ may be
used uninitialized in this function [-Wmaybe-uninitialized]
   g_free (*pp);
   ^~~~~~~~~~~~

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200206093238.203984-1-kuhn.chenqun@huawei.com>
[AJB: uses Thomas's single line allocation]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200225124710.14152-13-alex.bennee@linaro.org>