avformat/oggparseogm: check input size before reading t

Makes no difference in outcome, as the checks on t have no effect
when t was uninitialized

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7fa2b7b5d97c_3598_anOTHERS_DixX_in_Ogg_Sample.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/oggparseogm.c

@@ -126,6 +126,8 @@ ogm_dshow_header(AVFormatContext *s, int idx)
     if(*p != 1)
         return 1;
 
+    if (os->psize < 100)
+        return AVERROR_INVALIDDATA;
     t = AV_RL32(p + 96);
 
     if(t == 0x05589f80){