core: Ad ot_clear_gvariant(), use it

This saves us a conditional.

src/libotutil/ot-variant-utils.c

@@ -117,8 +117,7 @@ ot_util_variant_map (GFile *src,
   *out_variant = ret_variant;
   ret_variant = NULL;
  out:
-  if (ret_variant)
-    g_variant_unref (ret_variant);
+  ot_clear_gvariant (&ret_variant);
   if (mfile)
     g_mapped_file_unref (mfile);
   return ret;

src/libotutil/ot-variant-utils.h

@@ -27,6 +27,12 @@
 
 G_BEGIN_DECLS
 
+#define ot_clear_gvariant(a_v) do { \
+  if (*a_v)                         \
+    g_variant_unref (*a_v);         \
+  *a_v = NULL;                      \
+  } while (0);
+
 GHashTable *ot_util_variant_asv_to_hash_table (GVariant *variant);
 
 GVariant * ot_util_variant_take_ref (GVariant *variant);