mov: Avoid memcmp of uninitialised data

The string codec name need not be as long as the value we are
comparing it to, so memcmp may make decisions derived from
uninitialised data that valgrind then complains about (though the
overall result of the function will always be the same).  Use
strncmp instead, which will stop at the first zero byte and
therefore not encounter this issue.

libavformat/mov.c

@@ -1455,11 +1455,11 @@ static void mov_parse_stsd_video(MOVContext *c, AVIOContext *pb,
         av_dict_set(&st->metadata, "encoder", codec_name, 0);
 
     /* codec_tag YV12 triggers an UV swap in rawdec.c */
-    if (!memcmp(codec_name, "Planar Y'CbCr 8-bit 4:2:0", 25))
+    if (!strncmp(codec_name, "Planar Y'CbCr 8-bit 4:2:0", 25))
         st->codecpar->codec_tag = MKTAG('I', '4', '2', '0');
     /* Flash Media Server uses tag H.263 with Sorenson Spark */
     if (st->codecpar->codec_tag == MKTAG('H','2','6','3') &&
-        !memcmp(codec_name, "Sorenson H263", 13))
+        !strncmp(codec_name, "Sorenson H263", 13))
         st->codecpar->codec_id = AV_CODEC_ID_FLV1;
 
     st->codecpar->bits_per_coded_sample = avio_rb16(pb); /* depth */