testlibrary: Initialize autofree variable to silence a compiler warning

As with commit 43085c0e "dir: Consistently initialize g_autofree
variables", this is currently harmless because we never actually
early-return or goto out of the region between declaration and
initialization, but some compiler versions log a warning here anyway.

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit cffc3aed5b)

tests/testlibrary.c

@@ -207,7 +207,7 @@ test_installation_config (void)
   g_autofree char *path = NULL;
   g_autoptr(GFile) file = NULL;
   g_autoptr(GError) error = NULL;
-  g_autofree char *value;
+  g_autofree char *value = NULL;
   gboolean res;
   guint64 bytes;