g_object_get_property: Clarify meaning of G_TYPE_INVALID

"Uninitialized value" is partially correct, since it has not been
initialized with a type, but it's more precise to say
"zero-initialized value". It is still a programming error to pass a
pointer to uninitialized memory with arbitrary contents as the value.

Signed-off-by: Simon McVittie <smcv@collabora.com>

gobject/gobject.c

@@ -2607,7 +2607,7 @@ g_object_get_property (GObject	   *object,
       
       if (G_VALUE_TYPE (value) == G_TYPE_INVALID)
         {
-          /* uninitialized value */
+          /* zero-initialized value */
           g_value_init (value, pspec->value_type);
           prop_value = value;
         }