id3v2: strip trailing spaces from APIC tag

The APIC description must be unique, and some ID3v2 tag writers add
spaces to write several APIC entries with the same description. The
trailing spaces simply serve as a way to disambiguate the description.
Do this so that API users do not have to special-case mp3 to fix this
cosmetic issue.

Requested-by: wm4
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/id3v2.c

@@ -535,6 +535,13 @@ static void free_apic(void *obj)
     av_freep(&apic);
 }
 
+static void rstrip_spaces(char *buf)
+{
+    size_t len = strlen(buf);
+    while (len > 0 && buf[len - 1] == ' ')
+        buf[--len] = 0;
+}
+
 static void read_apic(AVFormatContext *s, AVIOContext *pb, int taglen,
                       const char *tag, ID3v2ExtraMeta **extra_meta,
                       int isv34)
@@ -608,6 +615,10 @@ static void read_apic(AVFormatContext *s, AVIOContext *pb, int taglen,
     new_extra->next = *extra_meta;
     *extra_meta     = new_extra;
 
+    // The description must be unique, and some ID3v2 tag writers add spaces
+    // to write several APIC entries with the same description.
+    rstrip_spaces(apic->description);
+
     return;
 
 fail: