WinTab: Some fixes and improvements by Jason Friedman

* Some crash fix for WinTabMex.c: Set ghWintab = NULL on shutdown.
* Some code reformatting for WinTabMex.c
* wintabfastLoop.m/wintabslowLoop.m: Improve error handling, return one
extra field.

-> WinTabMex for 32/64-Bit Matlab on Windows rebuilt.