Fix compiler warnings. The warnings can result in a broken build.
This patch fixes warnings such as:

In file included from /usr/include/glib-2.0/glib.h:111,
                 from ../tests/plugin/syscall.c:13:
../tests/plugin/syscall.c: In function ‘print_entry’:
/usr/include/glib-2.0/glib/glib-autocleanups.h:28:3: error: ‘out’ may be
       used uninitialized in this function [-Werror=maybe-uninitialized]
   g_free (*pp);
   ^~~~~~~~~~~~
../tests/plugin/syscall.c:82:23: note: ‘out’ was declared here
     g_autofree gchar *out;
                       ^~~
In file included from /usr/include/glib-2.0/glib.h:111,
                 from ../tests/plugin/syscall.c:13:
../tests/plugin/syscall.c: In function ‘vcpu_syscall_ret’:
/usr/include/glib-2.0/glib/glib-autocleanups.h:28:3: error: ‘out’ may be
        used uninitialized in this function [-Werror=maybe-uninitialized]
   g_free (*pp);
   ^~~~~~~~~~~~
../tests/plugin/syscall.c:73:27: note: ‘out’ was declared here
         g_autofree gchar *out;
                           ^~~
cc1: all warnings being treated as errors

Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20211128011551.2115468-1-juro.bystricky@intel.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211129140932.4115115-9-alex.bennee@linaro.org>