mi: Hush an almost certainly bogus warning

In file included from ../mi/miexpose.c:83:
../mi/miexpose.c: In function ‘miHandleExposures’:
../include/regionstr.h:174:22: warning: ‘expBox.y2’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     (_pReg)->extents = *(_pBox);
     ~~~~~~~~~~~~~~~~~^~~~~~~~~~
../mi/miexpose.c:139:12: note: ‘expBox.y2’ was declared here
     BoxRec expBox;
            ^~~~~~

etc. It's initialized if (extents), and then only read if (extents),
but gcc doesn't seem to figure that out. Whatever, bzero it to be
explicit.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Acked-by: Keith Packard <keithp@keithp.com>

mi/miexpose.c

@@ -136,7 +136,7 @@ miHandleExposures(DrawablePtr pSrcDrawable, DrawablePtr pDstDrawable,
                                    the window background
                                  */
     WindowPtr pSrcWin;
-    BoxRec expBox;
+    BoxRec expBox = { 0, };
     Bool extents;
 
     /* avoid work if we can */