codecparser: h264: initialize parsing structures

codecparser: h264: initialize parsing structures

Initialize to 0 these parse structures before filling them: GstH264SEIMessage,
GstH264NalUnit, GstH264PPS, GstH264SPS and GstH264SliceHdr.

When calling the functions which fill those structures, they may fail, leaving
unitialized those structures. This situation may lead to future problems, such
as a segmentation fault when freeing, for example.

This patch initializes to zero these structures, before filling them.

https://bugzilla.gnome.org/show_bug.cgi?id=755161