http: Properly initialize icy headers string

The icy_metadata_headers string never gets initialized, so,
during the first call to av_strlcatf() in parse_icy(),
strlen() will be called on a pointer to uninitialized memory.
At best this causes some garbage data to be left at the
start of the string.

By initializing icy_metadata_headers to the empty string, the
first call to strlen() will always return 0, so that data is
appended from the start of the string.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/http.c

@@ -405,6 +405,7 @@ static int parse_content_encoding(URLContext *h, const char *p)
 static int parse_icy(HTTPContext *s, const char *tag, const char *p)
 {
     int len = 4 + strlen(p) + strlen(tag);
+    int is_first = !s->icy_metadata_headers;
     int ret;
 
     if (s->icy_metadata_headers)
@@ -413,6 +414,9 @@ static int parse_icy(HTTPContext *s, const char *tag, const char *p)
     if ((ret = av_reallocp(&s->icy_metadata_headers, len)) < 0)
         return ret;
 
+    if (is_first)
+        *s->icy_metadata_headers = '\0';
+
     av_strlcatf(s->icy_metadata_headers, len, "%s: %s\n", tag, p);
 
     return 0;