avcodec/ass_split: Don't presume strlen to be >= 2

Fixes potential heap-buffer-overflow.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/ass_split.c

@@ -376,7 +376,7 @@ ASSSplitContext *ff_ass_split(const char *buf)
     ASSSplitContext *ctx = av_mallocz(sizeof(*ctx));
     if (!ctx)
         return NULL;
-    if (buf && !memcmp(buf, "\xef\xbb\xbf", 3)) // Skip UTF-8 BOM header
+    if (buf && !strncmp(buf, "\xef\xbb\xbf", 3)) // Skip UTF-8 BOM header
         buf += 3;
     ctx->current_section = -1;
     if (ass_split(ctx, buf) < 0) {