url: Handle relative urls starting with two slashes

This is defined by RFC 3986 section 5.4.1 to be handled this way.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/utils.c

@@ -3396,11 +3396,17 @@ void ff_make_absolute_url(char *buf, int size, const char *base,
             av_strlcpy(buf, base, size);
         sep = strstr(buf, "://");
         if (sep) {
+            /* Take scheme from base url */
+            if (rel[1] == '/') {
+                sep[1] = '\0';
+            } else {
+                /* Take scheme and host from base url */
                 sep += 3;
                 sep = strchr(sep, '/');
                 if (sep)
                     *sep = '\0';
             }
+        }
         av_strlcat(buf, rel, size);
         return;
     }