swtpm_setup: Initialized @argv in get_swtpm_capabilities()

The compiler (though wrongly) identifies that the @argv variable
inside of get_swtpm_capabilities() function cam be used
uninitialized. While this is a spurious warning, it's common
practice to initialize g_autofree variables to NULL.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
diff --git a/src/swtpm_setup/swtpm_setup.c b/src/swtpm_setup/swtpm_setup.c
index 1b528c8..3570235 100644
--- a/src/swtpm_setup/swtpm_setup.c
+++ b/src/swtpm_setup/swtpm_setup.c
@@ -937,7 +937,7 @@
     gchar *my_argv[] = { "--print-capabilities", is_tpm2 ? "--tpm2" : NULL, NULL };
     g_autofree gchar *logop = NULL;
     g_autoptr(GError) error = NULL;
-    g_autofree gchar **argv;
+    g_autofree gchar **argv = NULL;
     int exit_status = 0;
     gboolean success;
     int ret = 1;