[XrdXrootd] Fix time-of-check vs time-of-use race condition

Between the call to stat and the open, the file size may change.
First open the file, then use fstat with the file descriptor to
ensure consistency.

src/XrdXrootd/XrdXrootdPrepare.cc

@@ -258,14 +258,19 @@ int XrdXrootdPrepare::Open(const char *reqid, int &fsz)
    strcpy(path, (const char *)LogDir);
    strcpy(path+LogDirLen, reqid);
 
+// Open the file and return the file descriptor
+//
+   if ((fd = open((const char *)path, O_RDONLY)) < 0) return -errno;
+
 // Get the file size
 //
-   if (stat((const char *)path, &buf)) return -errno;
+   if (fstat(fd, &buf) != 0) {
+     close(fd);
+     return -errno;
+   }
+
    fsz = buf.st_size;
 
-// Open the file and return the file descriptor
-//
-   if ((fd = open((const char *)path, O_RDONLY)) < 0) return -errno;
    return fd;
 }