BUG#27141 - Calling tell(-1) under Windows causes assertion failure in Debug mode

Original problem was fixed by Magnus (see BUG25807).

Currently only windows debug build causes assertion failure. This patch assures
that my_tell gets correct file descriptor on any platform by DBUG_ASSERT macro.

mysys/my_seek.c

@@ -88,6 +88,7 @@ my_off_t my_tell(File fd, myf MyFlags __attribute__((unused)))
   os_off_t pos;
   DBUG_ENTER("my_tell");
   DBUG_PRINT("my",("Fd: %d  MyFlags: %d",fd, MyFlags));
+  DBUG_ASSERT(fd >= 0);
 #ifdef HAVE_TELL
   pos=tell(fd);
 #else