xrootdfs: check file descriptor before using it in xrootdfs wcache

If file open failed with ENOSYS (it can if the server responded with kXR_Unsupported), then file descriptor is not initialized.
As a result any attempt to write to the file gives a segfault. This check gives a EBADF instead.

src/XrdFfs/XrdFfsWcache.cc

@@ -161,6 +161,11 @@ ssize_t XrdFfsWcache_pwrite(int fd, char *buf, size_t len, off_t offset)
     ssize_t rc;
     char *bufptr;
     fd -= XrdFfsPosix_baseFD;
+    if (fd < 0)
+    {
+        errno = EBADF;
+        return -1;
+    }
 
 /* do not use caching under these cases */
     if (len > XrdFfsWcacheBufsize/2 || fd >= XrdFfsWcacheNFILES)