id3v2mux: ensure valid sentinal for gst_structure_get()

id3v2mux: ensure valid sentinal for gst_structure_get()

gst_structure_get() is declared with G_GNUC_NULL_TERMINATED, ie
__attribute__((__sentinel__)), which means gcc will generate a
warning if the last parameter passed to the function is not NULL
(where a valid NULL in this context is defined as zero with any
pointer type).

The C code callers to gst_structure_get() within gst-plugins-good
use the C NULL definition (ie ((void*)0)), which is a valid sentinel.

However gstid3v2mux.cc uses the C++ NULL definition (ie 0L), which
is not a valid sentinel without an explicit cast to a pointer type.

Upstream-Status: Pending

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>