riched20 regression fix

[Krzysztof Foltman]

The original patch used to crash some installers because of division by 
zero, detected by Marcus Meissner. This patch solves the problem 
temporarily, until all the scrollbar/repainting-related code will 
eventually get rewritten.

ChangeLog:

   * added a check for empty scrollbar range
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Index: dlls/riched20/paint.c
===================================================================
RCS file: /home/wine/wine/dlls/riched20/paint.c,v
retrieving revision 1.1
diff -u -r1.1 paint.c
--- dlls/riched20/paint.c	5 Mar 2005 11:19:14 -0000	1.1
+++ dlls/riched20/paint.c	5 Mar 2005 23:33:04 -0000
@@ -310,6 +310,7 @@
   GetScrollInfo(hWnd, SB_VERT, &si);
   
   if (ypos < 0) {
+    if (si.nMax<1) si.nMax = 1;
     perc = 1.0*si.nPos/si.nMax;
     ypos = perc*overflow;
   }