rtpbuffer: make sure header extension buffer is initialized

rtpbuffer: make sure header extension buffer is initialized

Based upon valgrind finding:

Conditional jump or move depends on uninitialised value(s)
   at 0x4AFF589: read_rtp_header_extensions (gstrtpbasedepayload.c:1197)
   by 0x4AFF9E5: gst_rtp_base_depayload_set_headers
(gstrtpbasedepayload.c:1298)
   by 0x4AFFEE0: gst_rtp_base_depayload_do_push
(gstrtpbasedepayload.c:1413)
   by 0x4AFFF53: gst_rtp_base_depayload_push
(gstrtpbasedepayload.c:1448)
   by 0x4AFDEBA: gst_rtp_base_depayload_handle_buffer
(gstrtpbasedepayload.c:801)
   by 0x4AFE41E: gst_rtp_base_depayload_chain_list
(gstrtpbasedepayload.c:899)
   by 0x48F262C: gst_pad_chain_data_unchecked (gstpad.c:4414)
   by 0x48F3333: gst_pad_push_data (gstpad.c:4655)
   by 0x48F3DF8: gst_pad_push_list (gstpad.c:4814)
   by 0x4AFAD87: gst_rtp_base_payload_push_list
(gstrtpbasepayload.c:1978)
   by 0x72B3154: gst_rtp_vp8_pay_handle_buffer (gstrtpvp8pay.c:672)
   by 0x4AF7031: gst_rtp_base_payload_chain (gstrtpbasepayload.c:868)
 Uninitialised value was created by a heap allocation
   at 0x483C77F: malloc (in
/usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x4B8BA78: g_malloc (gmem.c:106)
   by 0x4BA3A9D: g_slice_alloc (gslice.c:1069)
   by 0x488D777: _sysmem_new_block (gstallocator.c:413)
   by 0x488DB28: default_alloc (gstallocator.c:512)
   by 0x488D3E8: gst_allocator_alloc (gstallocator.c:310)
   by 0x4AE97E3: gst_rtp_buffer_set_extension_data (gstrtpbuffer.c:856)
   by 0x4AF9EC6: set_headers (gstrtpbasepayload.c:1757)
   by 0x489FE4D: gst_buffer_list_foreach (gstbufferlist.c:287)
   by 0x4AFA87A: gst_rtp_base_payload_prepare_push
(gstrtpbasepayload.c:1915)
   by 0x4AFAD06: gst_rtp_base_payload_push_list
(gstrtpbasepayload.c:1970)
   by 0x72B3154: gst_rtp_vp8_pay_handle_buffer (gstrtpvp8pay.c:672)

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1075>