r128, radeon, r200: Check ctx->WinSysDrawBuffer before calling function that dereferences it.

Same fix as for r300 (which fixed https://bugs.freedesktop.org/show_bug.cgi?id=10417),
since it's likely an issue with those drivers too.

src/mesa/drivers/dri/r128/r128_lock.c

@@ -52,8 +52,10 @@ static void
 r128UpdatePageFlipping( r128ContextPtr rmesa )
 {
    rmesa->doPageFlip = rmesa->sarea->pfAllowPageFlip;
-   driFlipRenderbuffers(rmesa->glCtx->WinSysDrawBuffer,
-                        rmesa->sarea->pfCurrentPage);
+   if (rmesa->glCtx->WinSysDrawBuffer) {
+      driFlipRenderbuffers(rmesa->glCtx->WinSysDrawBuffer,
+                           rmesa->sarea->pfCurrentPage);
+   }
    rmesa->new_state |= R128_NEW_WINDOW;
 }
 

src/mesa/drivers/dri/r200/r200_lock.c

@@ -53,8 +53,10 @@ static void
 r200UpdatePageFlipping( r200ContextPtr rmesa )
 {
    rmesa->doPageFlip = rmesa->sarea->pfState;
-   driFlipRenderbuffers(rmesa->glCtx->WinSysDrawBuffer,
-                        rmesa->sarea->pfCurrentPage);
+   if (rmesa->glCtx->WinSysDrawBuffer) {
+      driFlipRenderbuffers(rmesa->glCtx->WinSysDrawBuffer,
+                           rmesa->sarea->pfCurrentPage);
+   }
 }
 
 

src/mesa/drivers/dri/radeon/radeon_lock.c

@@ -56,8 +56,10 @@ static void
 radeonUpdatePageFlipping( radeonContextPtr rmesa )
 {
    rmesa->doPageFlip = rmesa->sarea->pfState;
-   driFlipRenderbuffers(rmesa->glCtx->WinSysDrawBuffer,
-                        rmesa->sarea->pfCurrentPage);
+   if (rmesa->glCtx->WinSysDrawBuffer) {
+      driFlipRenderbuffers(rmesa->glCtx->WinSysDrawBuffer,
+                           rmesa->sarea->pfCurrentPage);
+   }
 }