Silence REGION_INIT() warning.

Evaluating the address of a BoxRec as a boolean gives this warning:

  i830_driver.c:2317: warning: the address of 'ScreenBox' will always
  evaluate as 'true'

which is pretty annoying.  This patch compares the address to NULL to
avoid the pointer->bool conversion and gets rid of the warning.  Seems
like a lame hack, but the warning is worse.

include/regionstr.h

@@ -158,7 +158,7 @@ extern RegDataRec miBrokenData;
 
 #define REGION_INIT(_pScreen, _pReg, _rect, _size) \
 { \
-    if (_rect) \
+    if ((_rect) != NULL)				\
     { \
         (_pReg)->extents = *(_rect); \
         (_pReg)->data = (RegDataPtr)NULL; \