matroska: don't overwrite string values until read/alloc was succesful.

This prevents certain tags with a default value assigned to them (as per
the EBML syntax elements) from ever being assigned a NULL value. Other
parts of the code rely on these being non-NULL (i.e. they don't check for
NULL before e.g. using the string in strcmp() or similar), and thus in
effect this prevents crashes when reading of such specific tags fails,
either because of low memory or because of targeted file corruption.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavformat/matroskadec.c

@@ -639,16 +639,19 @@ static int ebml_read_float(AVIOContext *pb, int size, double *num)
  */
 static int ebml_read_ascii(AVIOContext *pb, int size, char **str)
 {
-    av_free(*str);
+    char *res;
+
     /* EBML strings are usually not 0-terminated, so we allocate one
      * byte more, read the string and NULL-terminate it ourselves. */
-    if (!(*str = av_malloc(size + 1)))
+    if (!(res = av_malloc(size + 1)))
         return AVERROR(ENOMEM);
-    if (avio_read(pb, (uint8_t *) *str, size) != size) {
-        av_freep(str);
+    if (avio_read(pb, (uint8_t *) res, size) != size) {
+        av_free(res);
         return AVERROR(EIO);
     }
-    (*str)[size] = '\0';
+    (res)[size] = '\0';
+    av_free(*str);
+    *str = res;
 
     return 0;
 }