Empty values are not valid GParamSpec

The validate() vfunc for GParamSpecParam returns FALSE for empty GValue,
which means the is_valid() vfunc should do the same.

This avoids a segfault when calling g_param_value_is_valid() on a
GParamSpecParam.

Fixes: #2770

gobject/gparamspecs.c

@@ -894,6 +894,9 @@ param_param_is_valid (GParamSpec   *pspec,
 {
   GParamSpec *param = value->data[0].v_pointer;
 
+  if (param == NULL)
+    return FALSE;
+
   return g_value_type_compatible (G_PARAM_SPEC_TYPE (param), G_PARAM_SPEC_VALUE_TYPE (pspec));
 }