lavf/file: file_check: Handle file URLs that start with "file:"

Handle the URL analog to file_open, it may contain a "file:"
prefix. Skip it. Make access checks to file URLs starting
with "file:" work.

Fix part of ticket #3249.
Signed-off-by: Alexander Strasser <eclipse7@gmx.net>

libavformat/file.c

@@ -104,25 +104,30 @@ static int file_get_handle(URLContext *h)
 
 static int file_check(URLContext *h, int mask)
 {
-#if HAVE_ACCESS && defined(R_OK)
     int ret = 0;
-    if (access(h->filename, F_OK) < 0)
+    const char *filename = h->filename;
+    av_strstart(filename, "file:", &filename);
+
+    {
+#if HAVE_ACCESS && defined(R_OK)
+    if (access(filename, F_OK) < 0)
         return AVERROR(errno);
     if (mask&AVIO_FLAG_READ)
-        if (access(h->filename, R_OK) >= 0)
+        if (access(filename, R_OK) >= 0)
             ret |= AVIO_FLAG_READ;
     if (mask&AVIO_FLAG_WRITE)
-        if (access(h->filename, W_OK) >= 0)
+        if (access(filename, W_OK) >= 0)
             ret |= AVIO_FLAG_WRITE;
 #else
     struct stat st;
-    int ret = stat(h->filename, &st);
+    ret = stat(filename, &st);
     if (ret < 0)
         return AVERROR(errno);
 
     ret |= st.st_mode&S_IRUSR ? mask&AVIO_FLAG_READ  : 0;
     ret |= st.st_mode&S_IWUSR ? mask&AVIO_FLAG_WRITE : 0;
 #endif
+    }
     return ret;
 }