Only consider -1 as an error return value for open().

This is:
 - what Posix says (-1 on error, >=0 on success)
 - fixing a bug on winCE (<0 sometimes)

Patch by Martin Storsjö: martin martin st

Originally committed as revision 19513 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/file.c

@@ -51,7 +51,7 @@ static int file_open(URLContext *h, const char *filename, int flags)
     access |= O_BINARY;
 #endif
     fd = open(filename, access, 0666);
-    if (fd < 0)
+    if (fd == -1)
         return AVERROR(ENOENT);
     h->priv_data = (void *) (intptr_t) fd;
     return 0;