lavf/id3v2: always strdup the value.

This simplifies the code but also fix a warning: ff_id3v1_genre_str
array contains const strings so do the string dup now instead of in
av_dict_set().

libavformat/id3v2.c

@@ -272,7 +272,7 @@ static int decode_str(AVFormatContext *s, AVIOContext *pb, int encoding,
 static void read_ttag(AVFormatContext *s, AVIOContext *pb, int taglen, const char *key)
 {
     uint8_t *dst;
-    int encoding, dict_flags = AV_DICT_DONT_OVERWRITE;
+    int encoding, dict_flags = AV_DICT_DONT_OVERWRITE | AV_DICT_DONT_STRDUP_VAL;
     unsigned genre;
 
     if (taglen < 1)
@@ -290,7 +290,7 @@ static void read_ttag(AVFormatContext *s, AVIOContext *pb, int taglen, const cha
         && (sscanf(dst, "(%d)", &genre) == 1 || sscanf(dst, "%d", &genre) == 1)
         && genre <= ID3v1_GENRE_MAX) {
         av_freep(&dst);
-        dst = ff_id3v1_genre_str[genre];
+        dst = av_strdup(ff_id3v1_genre_str[genre]);
     } else if (!(strcmp(key, "TXXX") && strcmp(key, "TXX"))) {
         /* dst now contains the key, need to get value */
         key = dst;
@@ -299,11 +299,8 @@ static void read_ttag(AVFormatContext *s, AVIOContext *pb, int taglen, const cha
             av_freep(&key);
             return;
         }
-        dict_flags |= AV_DICT_DONT_STRDUP_VAL | AV_DICT_DONT_STRDUP_KEY;
-    }
-    else if (*dst)
-        dict_flags |= AV_DICT_DONT_STRDUP_VAL;
-    else
+        dict_flags |= AV_DICT_DONT_STRDUP_KEY;
+    } else if (!*dst)
         av_freep(&dst);
 
     if (dst)