lavf/avio: check for : in filenames for protocols.

If the first "special" character in a filename is a comma,
it can introduce protocol options, but only if there is a
colon at the end. Otherwise, it is just a filename with a
comma.

Fix trac ticket #2303.
(cherry picked from commit d9fad53f4b)

libavformat/avio.c

@@ -296,7 +296,9 @@ int ffurl_alloc(URLContext **puc, const char *filename, int flags,
                                      "Missing call to av_register_all()?\n");
     }
 
-    if (filename[proto_len] != ':' &&  filename[proto_len] != ',' || is_dos_path(filename))
+    if (filename[proto_len] != ':' &&
+        (filename[proto_len] != ',' || !strchr(filename + proto_len + 1, ':')) ||
+        is_dos_path(filename))
         strcpy(proto_str, "file");
     else
         av_strlcpy(proto_str, filename, FFMIN(proto_len+1, sizeof(proto_str)));